perf: UITheme::getMetrics const and const-ref usage (#1094)

## Summary

**What is the goal of this PR?**

Small cleanup to make getTheme and getMetrics methods on UITheme const.
They return const refs, so updated call sites to use `const auto&`.

Realistically this won't make much performance difference, but it better
conveys the nature of theme metrics being shared const state.

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**NO**_

src/activities/home/HomeActivity.cpp

@@ -116,7 +116,7 @@ void HomeActivity::onEnter() {
 
   selectorIndex = 0;
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   loadRecentBooks(metrics.homeRecentBooksCount);
 
   // Trigger first update
@@ -212,7 +212,7 @@ void HomeActivity::loop() {
 }
 
 void HomeActivity::render(Activity::RenderLock&&) {
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 

src/activities/home/MyLibraryActivity.cpp

@@ -201,7 +201,7 @@ void MyLibraryActivity::render(Activity::RenderLock&&) {
 
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
 
   std::string folderName = (basepath == "/") ? tr(STR_SD_CARD) : basepath.substr(basepath.rfind('/') + 1);
   GUI.drawHeader(renderer, Rect{0, metrics.topPadding, pageWidth, metrics.headerHeight}, folderName.c_str());

src/activities/home/RecentBooksActivity.cpp

@@ -88,7 +88,7 @@ void RecentBooksActivity::render(Activity::RenderLock&&) {
 
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
 
   GUI.drawHeader(renderer, Rect{0, metrics.topPadding, pageWidth, metrics.headerHeight}, tr(STR_MENU_RECENT_BOOKS));
 

src/activities/network/CalibreConnectActivity.cpp

@@ -169,7 +169,7 @@ void CalibreConnectActivity::loop() {
 }
 
 void CalibreConnectActivity::render(Activity::RenderLock&&) {
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 

src/activities/network/CrossPointWebServerActivity.cpp

@@ -343,7 +343,7 @@ void CrossPointWebServerActivity::render(Activity::RenderLock&&) {
   // Subactivities handle their own rendering
   if (state == WebServerActivityState::SERVER_RUNNING || state == WebServerActivityState::AP_STARTING) {
     renderer.clearScreen();
-    auto metrics = UITheme::getInstance().getMetrics();
+    const auto& metrics = UITheme::getInstance().getMetrics();
     const auto pageWidth = renderer.getScreenWidth();
     const auto pageHeight = renderer.getScreenHeight();
 
@@ -386,7 +386,7 @@ void drawQRCode(const GfxRenderer& renderer, const int x, const int y, const std
 }
 
 void CrossPointWebServerActivity::renderServerRunning() const {
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
 
   GUI.drawHeader(renderer, Rect{0, metrics.topPadding, pageWidth, metrics.headerHeight},

src/activities/network/NetworkModeSelectionActivity.cpp

@@ -57,7 +57,7 @@ void NetworkModeSelectionActivity::loop() {
 void NetworkModeSelectionActivity::render(Activity::RenderLock&&) {
   renderer.clearScreen();
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 

src/activities/network/WifiSelectionActivity.cpp

@@ -471,7 +471,7 @@ void WifiSelectionActivity::render(Activity::RenderLock&&) {
 
   renderer.clearScreen();
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 
@@ -514,7 +514,7 @@ void WifiSelectionActivity::render(Activity::RenderLock&&) {
 }
 
 void WifiSelectionActivity::renderNetworkList() const {
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 

src/activities/reader/EpubReaderActivity.cpp

@@ -526,7 +526,7 @@ void EpubReaderActivity::render(Activity::RenderLock&& lock) {
   orientedMarginRight += SETTINGS.screenMargin;
   orientedMarginBottom += SETTINGS.screenMargin;
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
 
   // Add status bar margin
   if (SETTINGS.statusBar != CrossPointSettings::STATUS_BAR_MODE::NONE) {
@@ -713,7 +713,7 @@ void EpubReaderActivity::renderContents(std::unique_ptr<Page> page, const int or
 
 void EpubReaderActivity::renderStatusBar(const int orientedMarginRight, const int orientedMarginBottom,
                                          const int orientedMarginLeft) const {
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
 
   // determine visible status bar elements
   const bool showProgressPercentage = SETTINGS.statusBar == CrossPointSettings::STATUS_BAR_MODE::FULL;

src/activities/reader/TxtReaderActivity.cpp

@@ -139,7 +139,7 @@ void TxtReaderActivity::initializeReader() {
   orientedMarginRight += cachedScreenMargin;
   orientedMarginBottom += cachedScreenMargin;
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
 
   // Add status bar margin
   if (SETTINGS.statusBar != CrossPointSettings::STATUS_BAR_MODE::NONE) {
@@ -476,7 +476,7 @@ void TxtReaderActivity::renderStatusBar(const int orientedMarginRight, const int
   const bool showBatteryPercentage =
       SETTINGS.hideBatteryPercentage == CrossPointSettings::HIDE_BATTERY_PERCENTAGE::HIDE_NEVER;
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto screenHeight = renderer.getScreenHeight();
   // Adjust text position upward when progress bar is shown to avoid overlap
   const auto textY = screenHeight - orientedMarginBottom - 4;

src/activities/settings/ButtonRemapActivity.cpp

@@ -104,7 +104,7 @@ void ButtonRemapActivity::render(Activity::RenderLock&&) {
     return "-";
   };
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 

src/activities/settings/CalibreSettingsActivity.cpp

@@ -114,7 +114,7 @@ void CalibreSettingsActivity::handleSelection() {
 void CalibreSettingsActivity::render(Activity::RenderLock&&) {
   renderer.clearScreen();
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
   GUI.drawHeader(renderer, Rect{0, metrics.topPadding, pageWidth, metrics.headerHeight}, tr(STR_OPDS_BROWSER));

src/activities/settings/ClearCacheActivity.cpp

@@ -19,7 +19,7 @@ void ClearCacheActivity::onEnter() {
 void ClearCacheActivity::onExit() { ActivityWithSubactivity::onExit(); }
 
 void ClearCacheActivity::render(Activity::RenderLock&&) {
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 

src/activities/settings/KOReaderAuthActivity.cpp

@@ -91,7 +91,7 @@ void KOReaderAuthActivity::onExit() {
 void KOReaderAuthActivity::render(Activity::RenderLock&&) {
   renderer.clearScreen();
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 

src/activities/settings/KOReaderSettingsActivity.cpp

@@ -136,7 +136,7 @@ void KOReaderSettingsActivity::handleSelection() {
 void KOReaderSettingsActivity::render(Activity::RenderLock&&) {
   renderer.clearScreen();
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 

src/activities/settings/OtaUpdateActivity.cpp

@@ -84,7 +84,7 @@ void OtaUpdateActivity::render(Activity::RenderLock&&) {
     return;
   }
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 

src/activities/settings/SettingsActivity.cpp

@@ -216,7 +216,7 @@ void SettingsActivity::render(Activity::RenderLock&&) {
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
 
   GUI.drawHeader(renderer, Rect{0, metrics.topPadding, pageWidth, metrics.headerHeight}, tr(STR_SETTINGS_TITLE),
                  CROSSPOINT_VERSION);

src/activities/util/KeyboardEntryActivity.cpp

@@ -195,7 +195,7 @@ void KeyboardEntryActivity::render(Activity::RenderLock&&) {
 
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
 
   GUI.drawHeader(renderer, Rect{0, metrics.topPadding, pageWidth, metrics.headerHeight}, title.c_str());
 

src/components/UITheme.h

@@ -14,8 +14,8 @@ class UITheme {
   UITheme();
   static UITheme& getInstance() { return instance; }
 
-  const ThemeMetrics& getMetrics() { return *currentMetrics; }
+  const ThemeMetrics& getMetrics() const { return *currentMetrics; }
-  const BaseTheme& getTheme() { return *currentTheme; }
+  const BaseTheme& getTheme() const { return *currentTheme; }
   void reload();
   void setTheme(CrossPointSettings::UI_THEME type);
   static int getNumberOfItemsPerPage(const GfxRenderer& renderer, bool hasHeader, bool hasTabBar, bool hasButtonHints,

src/components/themes/BaseTheme.cpp

@@ -736,7 +736,7 @@ void BaseTheme::drawReadingProgressBar(const GfxRenderer& renderer, const size_t
 }
 
 void BaseTheme::drawHelpText(const GfxRenderer& renderer, Rect rect, const char* label) const {
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   auto truncatedLabel =
       renderer.truncatedText(SMALL_FONT_ID, label, rect.width - metrics.contentSidePadding * 2, EpdFontFamily::REGULAR);
   renderer.drawCenteredText(SMALL_FONT_ID, rect.y, truncatedLabel.c_str());