feat(audio): audio output device selection (closes #169)

Adds the ability to choose which audio output device Psysonic plays
through — useful for USB DACs, dedicated soundcards, and multi-device setups.

Rust (audio.rs):
- open_stream_for_device_and_rate(device_name, rate): opens a named
  device by cpal name, falls back to system default if not found
- AudioEngine.selected_device: persists the chosen device across
  stream reopens so hi-res rate switches stay on the right device
- audio_list_devices: Tauri command — returns all output device names
- audio_set_device: Tauri command — switches device immediately,
  drops old sinks, emits audio:device-changed
- start_device_watcher: now handles two cases:
    1. No pinned device + system default changed → reopen on new default
    2. Pinned device disappeared (DAC unplugged) → clear selected_device,
       fall back to system default, emit audio:device-reset

Frontend:
- authStore: audioOutputDevice (string | null), persisted
- playerStore: applies stored device on cold start
- App.tsx: listens to audio:device-reset, clears authStore device
  and restarts playback on the fallback device
- Settings → Audio tab: device dropdown at top (above Hi-Res and EQ),
  uses CustomSelect (portal-based, styled), all 8 locales
- CustomSelect: added disabled prop

Note: exclusive mode (WASAPI exclusive, CoreAudio exclusive) is out
of scope for now.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/store/authStore.ts

@@ -77,6 +77,8 @@ interface AuthState {
 
   /** Alpha: native hi-res sample rate output (disabled = safe 44.1 kHz mode) */
   enableHiRes: boolean;
+  /** Selected audio output device name. null = system default. */
+  audioOutputDevice: string | null;
 
   /** Alpha: ephemeral queue prefetch cache on disk */
   hotCacheEnabled: boolean;
@@ -197,6 +199,7 @@ interface AuthState {
   setLastSeenChangelogVersion: (v: string) => void;
   setSeekbarStyle: (v: SeekbarStyle) => void;
   setEnableHiRes: (v: boolean) => void;
+  setAudioOutputDevice: (v: string | null) => void;
   setHotCacheEnabled: (v: boolean) => void;
   setHotCacheMaxMb: (v: number) => void;
   setHotCacheDebounceSec: (v: number) => void;
@@ -289,6 +292,7 @@ export const useAuthStore = create<AuthState>()(
       lastSeenChangelogVersion: '',
       seekbarStyle: 'waveform',
       enableHiRes: false,
+      audioOutputDevice: null,
       hotCacheEnabled: false,
       hotCacheMaxMb: 256,
       hotCacheDebounceSec: 30,
@@ -410,6 +414,7 @@ export const useAuthStore = create<AuthState>()(
 
       setSeekbarStyle: (v) => set({ seekbarStyle: v }),
       setEnableHiRes: (v) => set({ enableHiRes: v }),
+      setAudioOutputDevice: (v) => set({ audioOutputDevice: v }),
       setHotCacheEnabled: (v) => set({ hotCacheEnabled: v }),
       setHotCacheMaxMb: (v) => set({ hotCacheMaxMb: v }),
       setHotCacheDebounceSec: (v) => set({ hotCacheDebounceSec: v }),