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/Epub/Epub/converters/JpegToFramebufferConverter.cpp

@@ -1,5 +1,6 @@
 #include "JpegToFramebufferConverter.h"
 
+#include <FsHelpers.h>
 #include <GfxRenderer.h>
 #include <HalStorage.h>
 #include <JPEGDEC.h>
@@ -486,9 +487,5 @@ bool JpegToFramebufferConverter::decodeToFramebuffer(const std::string& imagePat
 }
 
 bool JpegToFramebufferConverter::supportsFormat(const std::string& extension) {
-  std::string ext = extension;
-  for (auto& c : ext) {
-    c = tolower(c);
-  }
-  return (ext == ".jpg" || ext == ".jpeg");
+  return FsHelpers::hasJpgExtension(extension);
 }

lib/Epub/Epub/converters/PngToFramebufferConverter.cpp

@@ -1,5 +1,6 @@
 #include "PngToFramebufferConverter.h"
 
+#include <FsHelpers.h>
 #include <GfxRenderer.h>
 #include <HalStorage.h>
 #include <Logging.h>
@@ -391,9 +392,5 @@ bool PngToFramebufferConverter::decodeToFramebuffer(const std::string& imagePath
 }
 
 bool PngToFramebufferConverter::supportsFormat(const std::string& extension) {
-  std::string ext = extension;
-  for (auto& c : ext) {
-    c = tolower(c);
-  }
-  return (ext == ".png");
+  return FsHelpers::hasPngExtension(extension);
 }