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/types/settings.ts

@@ -78,6 +78,8 @@ export type AppSettings = {
 	theme: ThemeName;
 	fontSize: number;
 	playbackSpeed: number;
+	/** Playback volume 0–1 (default: 1 = 100%). */
+	volume: number;
 	downloadPath: string;
 	/** Render the app background transparent (let the terminal's own bg show). */
 	transparentBackground: boolean;