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/util/BookmarkStore.h

@@ -0,0 +1,24 @@
+#pragma once
+#include <cstdint>
+#include <string>
+#include <vector>
+
+struct Bookmark {
+  int16_t spineIndex;
+  int16_t pageNumber;
+  std::string snippet;  // First sentence or text excerpt from the page
+};
+
+class BookmarkStore {
+ public:
+  static std::vector<Bookmark> load(const std::string& cachePath);
+  static bool save(const std::string& cachePath, const std::vector<Bookmark>& bookmarks);
+  static bool addBookmark(const std::string& cachePath, int spineIndex, int page, const std::string& snippet = "");
+  static bool removeBookmark(const std::string& cachePath, int spineIndex, int page);
+  static bool hasBookmark(const std::string& cachePath, int spineIndex, int page);
+
+ private:
+  static std::string filePath(const std::string& cachePath);
+  static constexpr int MAX_BOOKMARKS = 200;
+  static constexpr int MAX_SNIPPET_LENGTH = 120;
+};