feat(player): persist volume across sessions, default to 100%

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).

src/utils/app-persistence.ts

@@ -32,6 +32,7 @@ const defaultSettings: AppSettings = {
 	theme: "system",
 	fontSize: 14,
 	playbackSpeed: 1,
+	volume: 1,
 	downloadPath: "",
 	transparentBackground: false,
 	showSelectionMarker: false,