feat: shared volume settings with a per-game override

- Music, sound and voice volumes are shared by all games by default; a game that
  has never been launched with its own volumes plays with the shared ones
- The sound section of the player settings has a "own volume for this game" switch:
  turning it on starts from the current values and then the sliders change only this
  game, turning it off removes the override and returns to the shared volumes
- The override belongs to the game (game_id from /api/versions/{id}), so forks and mods
  share it; it falls back to the version id when the id is unavailable
- Reading speed and auto delay stay shared
- volume-store.ts keeps the storage logic (damaged data ignored, values clamped,
  unavailable storage tolerated) and has its own tests

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

frontend/src/player/player.css

@@ -365,3 +365,7 @@
 .pg-input { display: flex; flex-direction: column; gap: 12px; align-items: center; pointer-events: auto; }
 .pg-input input { font: inherit; font-size: var(--pg-font, 24px); padding: 6px 10px; min-width: 40%; }
 .pg-caption { color: var(--pg-text, #fff); opacity: 0.8; text-align: center; padding: 4px 0; }
+
+/* checkbox row of the settings screen */
+.pg-check { display: flex; align-items: center; gap: 10px; margin: 4px 0 14px; cursor: pointer; font-size: 14px; }
+.pg-check input { width: 18px; height: 18px; accent-color: var(--pg-accent, #f5a040); }