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/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()) {