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

src/activities/ActivityManager.h

@@ -11,6 +11,7 @@
 
 #include "GfxRenderer.h"
 #include "MappedInputManager.h"
+#include "OpdsServerStore.h"
 
 class Activity;    // forward declaration
 class RenderLock;  // forward declaration
@@ -82,6 +83,7 @@ class ActivityManager {
   void goToFileBrowser(std::string path = {});
   void goToRecentBooks();
   void goToBrowser();
+  void goToBrowser(const struct OpdsServer& server);
   void goToReader(std::string path);
   void goToSleep();
   void goToBoot();