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**_

lib/FsHelpers/FsHelpers.cpp

@@ -1,8 +1,12 @@
 #include "FsHelpers.h"
 
+#include <cctype>
+#include <cstring>
 #include <vector>
 
-std::string FsHelpers::normalisePath(const std::string& path) {
+namespace FsHelpers {
+
+std::string normalisePath(const std::string& path) {
   std::vector<std::string> components;
   std::string component;
 
@@ -37,3 +41,41 @@ std::string FsHelpers::normalisePath(const std::string& path) {
 
   return result;
 }
+
+bool checkFileExtension(std::string_view fileName, const char* extension) {
+  const size_t extLen = strlen(extension);
+  if (fileName.length() < extLen) {
+    return false;
+  }
+
+  const size_t offset = fileName.length() - extLen;
+  for (size_t i = 0; i < extLen; i++) {
+    if (tolower(static_cast<unsigned char>(fileName[offset + i])) !=
+        tolower(static_cast<unsigned char>(extension[i]))) {
+      return false;
+    }
+  }
+  return true;
+}
+
+bool hasJpgExtension(std::string_view fileName) {
+  return checkFileExtension(fileName, ".jpg") || checkFileExtension(fileName, ".jpeg");
+}
+
+bool hasPngExtension(std::string_view fileName) { return checkFileExtension(fileName, ".png"); }
+
+bool hasBmpExtension(std::string_view fileName) { return checkFileExtension(fileName, ".bmp"); }
+
+bool hasGifExtension(std::string_view fileName) { return checkFileExtension(fileName, ".gif"); }
+
+bool hasEpubExtension(std::string_view fileName) { return checkFileExtension(fileName, ".epub"); }
+
+bool hasXtcExtension(std::string_view fileName) {
+  return checkFileExtension(fileName, ".xtc") || checkFileExtension(fileName, ".xtch");
+}
+
+bool hasTxtExtension(std::string_view fileName) { return checkFileExtension(fileName, ".txt"); }
+
+bool hasMarkdownExtension(std::string_view fileName) { return checkFileExtension(fileName, ".md"); }
+
+}  // namespace FsHelpers