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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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