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

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

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;