feat(player): video chapters & bookmarks with per-video persistence

[mazen-salah]

Summary

Adds a Chapters / bookmarks feature to the player, built on the current Jetpack Compose UI.

This supersedes #1146, which was written against the old XML / view-binding player and is no longer mergeable after the Compose rewrite — every layout it modified (exo_player_control_view.xml, bottom_sheet_sections.xml, …) has since been removed. I re-implemented the feature from scratch following the patterns already used by the existing selector panels.

What it does

• A Chapters button in the bottom player controls:
  • Tap → opens a chapters panel (same overlay style as the playback-speed / playlist selectors).
  • Long-press → adds a bookmark at the current position.
• In the panel you can tap an entry to seek to it, add a bookmark, import a chapters file, or export the current list. Entries are simple { "title": String, "timestampMs": Long } JSON objects.
• Chapters are persisted per video in the existing Room media_state table, so they survive reopening the file.

Implementation

• New Chapter model + ChapterListConverter in core:model (kotlinx-serialization, already a dependency there — keeps other modules dependency-free).
• media_state gains a chapters TEXT column; DB version 4 → 5 with an additive, non-destructive MIGRATION_4_5. Generated 5.json schema is included.
• MediaRepository gains updateChapters / getChaptersStream, using the same read-copy-upsert pattern as the other per-video state.
• UI follows the existing selector convention: OverlayView.CHAPTERS + ChaptersSelectorView + a rememberChaptersState holder, plus one PlayerButton in ControlsBottomView.
• No new dependencies and no DI-graph changes beyond the added repository method.

Testing

• Builds clean — ./gradlew :app:assembleDebug passes (Room v4→5 KSP, Hilt, resource linking and packaging all succeed).
• On-device verification of the full flow (add → seek → import/export → persistence across reopen) is in progress.

Happy to adjust the button placement, naming, or the import/export format to match your preferences. Supersedes #1146.