feat: Implement bookmark functionality for epub reader

Replace bookmark stubs with full add/remove/navigate implementation:

- BookmarkStore: per-book binary persistence on SD card with v2 format
  supporting text snippets (backward-compatible with v1)
- Visual bookmark ribbon indicator drawn on bookmarked pages via fillPolygon
- Reader menu dynamically shows Add/Remove Bookmark based on current page state
- Bookmark selection activity with chapter name, first sentence snippet, and
  page number display; long-press to delete with confirmation
- Go to Bookmark falls back to Table of Contents when no bookmarks exist
- Smart snippet extraction: skips partial sentences (lowercase first word)
  to capture the first full sentence on the page
- Label truncation reserves space for page suffix so it's never cut off
- Half refresh forced on menu exit to clear popup/menu artifacts

Co-authored-by: Cursor <cursoragent@cursor.com>

src/activities/reader/EpubReaderMenuActivity.h

@@ -16,6 +16,7 @@ class EpubReaderMenuActivity final : public ActivityWithSubactivity {
   // Menu actions available from the reader menu.
   enum class MenuAction {
     ADD_BOOKMARK,
+    REMOVE_BOOKMARK,
     LOOKUP,
     LOOKED_UP_WORDS,
     ROTATE_SCREEN,
@@ -31,10 +32,11 @@ class EpubReaderMenuActivity final : public ActivityWithSubactivity {
   explicit EpubReaderMenuActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, const std::string& title,
                                   const int currentPage, const int totalPages, const int bookProgressPercent,
                                   const uint8_t currentOrientation, const bool hasDictionary,
+                                  const bool isBookmarked,
                                   const std::function<void(uint8_t)>& onBack,
                                   const std::function<void(MenuAction)>& onAction)
       : ActivityWithSubactivity("EpubReaderMenu", renderer, mappedInput),
-        menuItems(buildMenuItems(hasDictionary)),
+        menuItems(buildMenuItems(hasDictionary, isBookmarked)),
         title(title),
         pendingOrientation(currentOrientation),
         currentPage(currentPage),
@@ -70,9 +72,13 @@ class EpubReaderMenuActivity final : public ActivityWithSubactivity {
   const std::function<void(uint8_t)> onBack;
   const std::function<void(MenuAction)> onAction;
 
-  static std::vector<MenuItem> buildMenuItems(bool hasDictionary) {
+  static std::vector<MenuItem> buildMenuItems(bool hasDictionary, bool isBookmarked) {
     std::vector<MenuItem> items;
-    items.push_back({MenuAction::ADD_BOOKMARK, "Add Bookmark"});
+    if (isBookmarked) {
+      items.push_back({MenuAction::REMOVE_BOOKMARK, "Remove Bookmark"});
+    } else {
+      items.push_back({MenuAction::ADD_BOOKMARK, "Add Bookmark"});
+    }
     if (hasDictionary) {
       items.push_back({MenuAction::LOOKUP, "Lookup Word"});
       items.push_back({MenuAction::LOOKED_UP_WORDS, "Lookup Word History"});