Store the playback volume in app settings (config.json) whenever it
changes and re-apply the previous session's level at boot, instead of
always starting at the old 70% fallback.
- AppSettings gains volume (default 1 = 100%); both default-settings
copies and the volume signal default are raised from 0.7 to 1.
- doSetVolume persists via the app store (mirrors playbackSpeed).
- The boot sync awaits the app store's async config load (new
whenReady()) so a persisted level is applied even when settings load
finishes after useAudio mounts.
- tests/volume-persistence.test.ts: default, clamp, and cross-session
reuse (fresh module instance simulates the next launch).
