feat: invalidate cache on web uploads and opds downloads and add Clear Cache action (#393)

## Summary

When uploading or downloading an updated ebook from SD/WebUI/OPDS with
same the filename the `.crosspoint` cache is not cleared. This can lead
to issues with the Table of Contents and hangs when switching between
chapters.

I encountered this issue in two places:
- When I need to do further ePub cleaning using Calibre after I load an
ePub and find that some of its formatting should be cleaned up. When I
reprocess the same book and want to place it back in the same location I
need a way to invalidate the cache.
- When syncing RSS feed generated epubs. I generate news ePubs with
filenames like `news-outlet.epub` and so every day when I fetch new news
the crosspoint cache needs to be cleared to load that file.

This change offers the following features:
- On web uploads, if the file already exists, the cache for that file is
cleared
- On OPDS downloads, if the file already exists, the cache for that file
is cleared
- There's now an action for `Clear Cache` in the Settings page which can
clear the cache for all books


Addresses
https://github.com/crosspoint-reader/crosspoint-reader/issues/281

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? PARTIALLY

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

src/util/StringUtils.cpp

@@ -49,6 +49,18 @@ bool checkFileExtension(const std::string& fileName, const char* extension) {
   return true;
 }
 
+bool checkFileExtension(const String& fileName, const char* extension) {
+  if (fileName.length() < strlen(extension)) {
+    return false;
+  }
+
+  String localFile(fileName);
+  String localExtension(extension);
+  localFile.toLowerCase();
+  localExtension.toLowerCase();
+  return localFile.endsWith(localExtension);
+}
+
 size_t utf8RemoveLastChar(std::string& str) {
   if (str.empty()) return 0;
   size_t pos = str.size() - 1;

src/util/StringUtils.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <WString.h>
+
 #include <string>
 
 namespace StringUtils {
@@ -15,6 +17,7 @@ std::string sanitizeFilename(const std::string& name, size_t maxLength = 100);
  * Check if the given filename ends with the specified extension (case-insensitive).
  */
 bool checkFileExtension(const std::string& fileName, const char* extension);
+bool checkFileExtension(const String& fileName, const char* extension);
 
 // UTF-8 safe string truncation - removes one character from the end
 // Returns the new size after removing one UTF-8 character