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>
