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)**.

---

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

src/RecentBooksStore.cpp

@@ -1,6 +1,7 @@
 #include "RecentBooksStore.h"
 
 #include <Epub.h>
+#include <FsHelpers.h>
 #include <HalStorage.h>
 #include <JsonSettingsIO.h>
 #include <Logging.h>
@@ -9,8 +10,6 @@
 
 #include <algorithm>
 
-#include "util/StringUtils.h"
-
 namespace {
 constexpr uint8_t RECENT_BOOKS_FILE_VERSION = 3;
 constexpr char RECENT_BOOKS_FILE_BIN[] = "/.crosspoint/recent.bin";
@@ -71,19 +70,17 @@ RecentBook RecentBooksStore::getDataFromBook(std::string path) const {
   // If epub, try to load the metadata for title/author and cover.
   // Use buildIfMissing=false to avoid heavy epub loading on boot; getTitle()/getAuthor() may be
   // blank until the book is opened, and entries with missing title are omitted from recent list.
-  if (StringUtils::checkFileExtension(lastBookFileName, ".epub")) {
+  if (FsHelpers::hasEpubExtension(lastBookFileName)) {
     Epub epub(path, "/.crosspoint");
     epub.load(false, true);
     return RecentBook{path, epub.getTitle(), epub.getAuthor(), epub.getThumbBmpPath()};
-  } else if (StringUtils::checkFileExtension(lastBookFileName, ".xtch") ||
-             StringUtils::checkFileExtension(lastBookFileName, ".xtc")) {
+  } else if (FsHelpers::hasXtcExtension(lastBookFileName)) {
     // Handle XTC file
     Xtc xtc(path, "/.crosspoint");
     if (xtc.load()) {
       return RecentBook{path, xtc.getTitle(), xtc.getAuthor(), xtc.getThumbBmpPath()};
     }
-  } else if (StringUtils::checkFileExtension(lastBookFileName, ".txt") ||
-             StringUtils::checkFileExtension(lastBookFileName, ".md")) {
+  } else if (FsHelpers::hasTxtExtension(lastBookFileName) || FsHelpers::hasMarkdownExtension(lastBookFileName)) {
     return RecentBook{path, lastBookFileName, "", ""};
   }
   return RecentBook{path, "", "", ""};