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.cpp

@@ -103,14 +103,11 @@ bool Epub::parseContentOpf(BookMetadataCache::BookMetadata& bookMetadata) {
           pos += strlen(pattern);
           const auto endPos = coverPageHtml.find('"', pos);
           if (endPos != std::string::npos) {
-            const auto ref = coverPageHtml.substr(pos, endPos - pos);
+            const auto ref = std::string_view{coverPageHtml}.substr(pos, endPos - pos);
             // Check if it's an image file
-            if (ref.length() >= 4) {
-              const auto ext = ref.substr(ref.length() - 4);
-              if (ext == ".png" || ext == ".jpg" || ext == "jpeg" || ext == ".gif") {
-                imageRef = ref;
-                break;
-              }
+            if (FsHelpers::hasPngExtension(ref) || FsHelpers::hasJpgExtension(ref) || FsHelpers::hasGifExtension(ref)) {
+              imageRef = ref;
+              break;
             }
           }
           pos = coverPageHtml.find(pattern, pos);
@@ -541,8 +538,7 @@ bool Epub::generateCoverBmp(bool cropped) const {
     return false;
   }
 
-  if (coverImageHref.substr(coverImageHref.length() - 4) == ".jpg" ||
-      coverImageHref.substr(coverImageHref.length() - 5) == ".jpeg") {
+  if (FsHelpers::hasJpgExtension(coverImageHref)) {
     LOG_DBG("EBP", "Generating BMP from JPG cover image (%s mode)", cropped ? "cropped" : "fit");
     const auto coverJpgTempPath = getCachePath() + "/.cover.jpg";
 
@@ -575,7 +571,7 @@ bool Epub::generateCoverBmp(bool cropped) const {
     return success;
   }
 
-  if (coverImageHref.substr(coverImageHref.length() - 4) == ".png") {
+  if (FsHelpers::hasPngExtension(coverImageHref)) {
     LOG_DBG("EBP", "Generating BMP from PNG cover image (%s mode)", cropped ? "cropped" : "fit");
     const auto coverPngTempPath = getCachePath() + "/.cover.png";
 
@@ -629,8 +625,7 @@ bool Epub::generateThumbBmp(int height) const {
   const auto coverImageHref = bookMetadataCache->coreMetadata.coverItemHref;
   if (coverImageHref.empty()) {
     LOG_DBG("EBP", "No known cover image for thumbnail");
-  } else if (coverImageHref.substr(coverImageHref.length() - 4) == ".jpg" ||
-             coverImageHref.substr(coverImageHref.length() - 5) == ".jpeg") {
+  } else if (FsHelpers::hasJpgExtension(coverImageHref)) {
     LOG_DBG("EBP", "Generating thumb BMP from JPG cover image");
     const auto coverJpgTempPath = getCachePath() + "/.cover.jpg";
 
@@ -666,7 +661,7 @@ bool Epub::generateThumbBmp(int height) const {
     }
     LOG_DBG("EBP", "Generated thumb BMP from JPG cover image, success: %s", success ? "yes" : "no");
     return success;
-  } else if (coverImageHref.substr(coverImageHref.length() - 4) == ".png") {
+  } else if (FsHelpers::hasPngExtension(coverImageHref)) {
     LOG_DBG("EBP", "Generating thumb BMP from PNG cover image");
     const auto coverPngTempPath = getCachePath() + "/.cover.png";
 

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);
 }