perf: Avoid creating strings for file extension checks (#1303)

## Summary

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

This change avoids the pattern of creating a `std::string` using
`.substr` in order to compare against a file extension literal.
```c++
std::string path;
if (path.length() >= 4 && path.substr(path.length() - 4) == ".ext")
```

The `checkFileExtension` utility has moved from StringUtils to
FsHelpers, to be available to code in lib/. The signature now accepts a
`std::string_view` instead of `std::string`, which makes the single
implementation reusable for Arduino `String`.

Added utility functions for commonly repeated extensions.

These changes **save about 2 KB of flash (5,999,427 to 5,997,343)**.

---

### AI Usage

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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/boot_sleep/SleepActivity.cpp

@@ -1,6 +1,7 @@
 #include "SleepActivity.h"
 
 #include <Epub.h>
+#include <FsHelpers.h>
 #include <GfxRenderer.h>
 #include <HalStorage.h>
 #include <I18n.h>
@@ -12,7 +13,6 @@
 #include "components/UITheme.h"
 #include "fontIds.h"
 #include "images/Logo120.h"
-#include "util/StringUtils.h"
 
 void SleepActivity::onEnter() {
   Activity::onEnter();
@@ -61,7 +61,7 @@ void SleepActivity::renderCustomSleepScreen() const {
         continue;
       }
 
-      if (filename.substr(filename.length() - 4) != ".bmp") {
+      if (!FsHelpers::hasBmpExtension(filename)) {
         LOG_DBG("SLP", "Skipping non-.bmp file name: %s", name);
         file.close();
         continue;
@@ -228,8 +228,7 @@ void SleepActivity::renderCoverSleepScreen() const {
   bool cropped = SETTINGS.sleepScreenCoverMode == CrossPointSettings::SLEEP_SCREEN_COVER_MODE::CROP;
 
   // Check if the current book is XTC, TXT, or EPUB
-  if (StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".xtc") ||
-      StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".xtch")) {
+  if (FsHelpers::hasXtcExtension(APP_STATE.openEpubPath)) {
     // Handle XTC file
     Xtc lastXtc(APP_STATE.openEpubPath, "/.crosspoint");
     if (!lastXtc.load()) {
@@ -243,7 +242,7 @@ void SleepActivity::renderCoverSleepScreen() const {
     }
 
     coverBmpPath = lastXtc.getCoverBmpPath();
-  } else if (StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".txt")) {
+  } else if (FsHelpers::hasTxtExtension(APP_STATE.openEpubPath)) {
     // Handle TXT file - looks for cover image in the same folder
     Txt lastTxt(APP_STATE.openEpubPath, "/.crosspoint");
     if (!lastTxt.load()) {
@@ -257,7 +256,7 @@ void SleepActivity::renderCoverSleepScreen() const {
     }
 
     coverBmpPath = lastTxt.getCoverBmpPath();
-  } else if (StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".epub")) {
+  } else if (FsHelpers::hasEpubExtension(APP_STATE.openEpubPath)) {
     // Handle EPUB file
     Epub lastEpub(APP_STATE.openEpubPath, "/.crosspoint");
     // Skip loading css since we only need metadata here

src/activities/home/FileBrowserActivity.cpp

@@ -1,6 +1,7 @@
 #include "FileBrowserActivity.h"
 
 #include <Epub.h>
+#include <FsHelpers.h>
 #include <GfxRenderer.h>
 #include <HalStorage.h>
 #include <I18n.h>
@@ -11,7 +12,6 @@
 #include "MappedInputManager.h"
 #include "components/UITheme.h"
 #include "fontIds.h"
-#include "util/StringUtils.h"
 
 namespace {
 constexpr unsigned long GO_HOME_MS = 1000;
@@ -91,10 +91,10 @@ void FileBrowserActivity::loadFiles() {
     if (file.isDirectory()) {
       files.emplace_back(std::string(name) + "/");
     } else {
-      auto filename = std::string(name);
-      if (StringUtils::checkFileExtension(filename, ".epub") || StringUtils::checkFileExtension(filename, ".xtch") ||
-          StringUtils::checkFileExtension(filename, ".xtc") || StringUtils::checkFileExtension(filename, ".txt") ||
-          StringUtils::checkFileExtension(filename, ".md") || StringUtils::checkFileExtension(filename, ".bmp")) {
+      std::string_view filename{name};
+      if (FsHelpers::hasEpubExtension(filename) || FsHelpers::hasXtcExtension(filename) ||
+          FsHelpers::hasTxtExtension(filename) || FsHelpers::hasMarkdownExtension(filename) ||
+          FsHelpers::hasBmpExtension(filename)) {
         files.emplace_back(filename);
       }
     }
@@ -120,7 +120,7 @@ void FileBrowserActivity::onExit() {
 
 void FileBrowserActivity::clearFileMetadata(const std::string& fullPath) {
   // Only clear cache for .epub files
-  if (StringUtils::checkFileExtension(fullPath, ".epub")) {
+  if (FsHelpers::hasEpubExtension(fullPath)) {
     Epub(fullPath, "/.crosspoint").clearCache();
     LOG_DBG("FileBrowser", "Cleared metadata cache for: %s", fullPath.c_str());
   }

src/activities/home/HomeActivity.cpp

@@ -2,6 +2,7 @@
 
 #include <Bitmap.h>
 #include <Epub.h>
+#include <FsHelpers.h>
 #include <GfxRenderer.h>
 #include <HalStorage.h>
 #include <I18n.h>
@@ -17,7 +18,6 @@
 #include "RecentBooksStore.h"
 #include "components/UITheme.h"
 #include "fontIds.h"
-#include "util/StringUtils.h"
 
 int HomeActivity::getMenuItemCount() const {
   int count = 4;  // File Browser, Recents, File transfer, Settings
@@ -61,7 +61,7 @@ void HomeActivity::loadRecentCovers(int coverHeight) {
       std::string coverPath = UITheme::getCoverThumbPath(book.coverBmpPath, coverHeight);
       if (!Storage.exists(coverPath.c_str())) {
         // If epub, try to load the metadata for title/author and cover
-        if (StringUtils::checkFileExtension(book.path, ".epub")) {
+        if (FsHelpers::hasEpubExtension(book.path)) {
           Epub epub(book.path, "/.crosspoint");
           // Skip loading css since we only need metadata here
           epub.load(false, true);
@@ -79,8 +79,7 @@ void HomeActivity::loadRecentCovers(int coverHeight) {
           }
           coverRendered = false;
           requestUpdate();
-        } else if (StringUtils::checkFileExtension(book.path, ".xtch") ||
-                   StringUtils::checkFileExtension(book.path, ".xtc")) {
+        } else if (FsHelpers::hasXtcExtension(book.path)) {
           // Handle XTC file
           Xtc xtc(book.path, "/.crosspoint");
           if (xtc.load()) {

src/activities/home/RecentBooksActivity.cpp

@@ -10,7 +10,6 @@
 #include "RecentBooksStore.h"
 #include "components/UITheme.h"
 #include "fontIds.h"
-#include "util/StringUtils.h"
 
 namespace {
 constexpr unsigned long GO_HOME_MS = 1000;

src/activities/reader/ReaderActivity.cpp

@@ -1,5 +1,6 @@
 #include "ReaderActivity.h"
 
+#include <FsHelpers.h>
 #include <HalStorage.h>
 
 #include "CrossPointSettings.h"
@@ -11,7 +12,6 @@
 #include "XtcReaderActivity.h"
 #include "activities/util/BmpViewerActivity.h"
 #include "activities/util/FullScreenMessageActivity.h"
-#include "util/StringUtils.h"
 
 std::string ReaderActivity::extractFolderPath(const std::string& filePath) {
   const auto lastSlash = filePath.find_last_of('/');
@@ -21,16 +21,14 @@ std::string ReaderActivity::extractFolderPath(const std::string& filePath) {
   return filePath.substr(0, lastSlash);
 }
 
-bool ReaderActivity::isXtcFile(const std::string& path) {
-  return StringUtils::checkFileExtension(path, ".xtc") || StringUtils::checkFileExtension(path, ".xtch");
-}
+bool ReaderActivity::isXtcFile(const std::string& path) { return FsHelpers::hasXtcExtension(path); }
 
 bool ReaderActivity::isTxtFile(const std::string& path) {
-  return StringUtils::checkFileExtension(path, ".txt") ||
-         StringUtils::checkFileExtension(path, ".md");  // Treat .md as txt files (until we have a markdown reader)
+  return FsHelpers::hasTxtExtension(path) ||
+         FsHelpers::hasMarkdownExtension(path);  // Treat .md as txt files (until we have a markdown reader)
 }
 
-bool ReaderActivity::isBmpFile(const std::string& path) { return StringUtils::checkFileExtension(path, ".bmp"); }
+bool ReaderActivity::isBmpFile(const std::string& path) { return FsHelpers::hasBmpExtension(path); }
 
 std::unique_ptr<Epub> ReaderActivity::loadEpub(const std::string& path) {
   if (!Storage.exists(path.c_str())) {