mod: Phase 3 — Re-port unmerged upstream PRs

Re-applied upstream PRs not yet merged to upstream/master:

- #1055: Byte-level framebuffer writes (fillPhysicalHSpan*,
  optimized fillRect/drawLine/fillRectDither/fillPolygon)
- #1027: Word-width cache (FNV-1a, 128-entry) and hyphenation
  early exit in ParsedText for 7-9% layout speedup
- #1068: Already present in upstream — URL hyphenation fix
- #1019: Already present in upstream — file extensions in browser
- #1090/#1185/#1217: KOReader sync improvements — binary credential
  store, document hash caching, ChapterXPathIndexer integration
- #1209: OPDS multi-server — OpdsBookBrowserActivity accepts
  OpdsServer, directory picker for downloads, download-complete
  prompt with open/back options
- #857: Dictionary activities already ported in Phase 1/2
- #1003: Placeholder cover already integrated in Phase 2

Also fixed: STR_OFF i18n string, include paths, replaced
Epub::isValidThumbnailBmp with Storage.exists, replaced
StringUtils::checkFileExtension with FsHelpers equivalents.

Made-with: Cursor

lib/KOReaderSync/KOReaderCredentialStore.h

@@ -8,17 +8,10 @@ enum class DocumentMatchMethod : uint8_t {
   BINARY = 1,    // Match by partial MD5 of file content (more accurate, but files must be identical)
 };
 
-class KOReaderCredentialStore;
-namespace JsonSettingsIO {
-bool saveKOReader(const KOReaderCredentialStore& store, const char* path);
-bool loadKOReader(KOReaderCredentialStore& store, const char* json, bool* needsResave);
-}  // namespace JsonSettingsIO
-
 /**
  * Singleton class for storing KOReader sync credentials on the SD card.
- * Passwords are XOR-obfuscated with the device's unique hardware MAC address
- * and base64-encoded before writing to JSON (not cryptographically secure,
- * but prevents casual reading and ties credentials to the specific device).
+ * Credentials are stored in /sd/.crosspoint/koreader.bin with basic
+ * XOR obfuscation to prevent casual reading (not cryptographically secure).
  */
 class KOReaderCredentialStore {
  private:
@@ -31,10 +24,8 @@ class KOReaderCredentialStore {
   // Private constructor for singleton
   KOReaderCredentialStore() = default;
 
-  bool loadFromBinaryFile();
-
-  friend bool JsonSettingsIO::saveKOReader(const KOReaderCredentialStore&, const char*);
-  friend bool JsonSettingsIO::loadKOReader(KOReaderCredentialStore&, const char*, bool*);
+  // XOR obfuscation (symmetric - same for encode/decode)
+  void obfuscate(std::string& data) const;
 
  public:
   // Delete copy constructor and assignment