feat(audio): bit-perfect hi-res playback + underrun hardening (opt-in alpha)

Safe mode (default): all audio outputs at 44.1 kHz, rodio resamples
internally. Maximum stability out of the box.

Hi-Res mode (alpha toggle): stream opens at the file's native sample rate
(e.g. 88.2/96/192 kHz) for bit-perfect output.

Rust (audio.rs):
- open_stream_for_rate(): CPAL queries device supported configs, finds
  exact rate match or falls back to highest available / device default.
- create_engine() thread: ThreadPriority::Max (silently ignored without
  CAP_SYS_NICE), loop handles rate-switch requests, PIPEWIRE_LATENCY
  scales with rate (8192 frames for >48 kHz ≈ 93 ms quantum), keeps
  PULSE_LATENCY_MSEC in sync.
- audio_play(): hi_res_enabled param gates effective_rate (44100 vs
  native); stream re-opens only when needed; 150 ms PipeWire settle +
  500 ms sink pre-fill (pause→append→sleep→play) for hi-res tracks.
- audio_chain_preload(): hi_res_enabled param; rate-mismatch bail skipped
  in safe mode so gapless chains always work at 44.1 kHz.
- SizedDecoder MSS buffer: 4 MB (was 512 KB) to reduce Symphonia read
  overhead on high-bitrate hi-res files.
- thread-priority crate added to Cargo.toml.

Frontend:
- authStore: enableHiRes (bool, default false) + setEnableHiRes.
- playerStore: hiResEnabled passed to all audio_play /
  audio_chain_preload invoke calls.
- Settings → Audio tab: "Native Hi-Res Playback" toggle with Alpha
  badge and stability warning, i18n for all 7 languages.

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

src/store/authStore.ts

@@ -47,6 +47,9 @@ interface AuthState {
   showChangelogOnUpdate: boolean;
   lastSeenChangelogVersion: string;
 
+  /** Alpha: native hi-res sample rate output (disabled = safe 44.1 kHz mode) */
+  enableHiRes: boolean;
+
   /** Alpha: ephemeral queue prefetch cache on disk */
   hotCacheEnabled: boolean;
   hotCacheMaxMb: number;
@@ -95,6 +98,7 @@ interface AuthState {
   setShowFullscreenLyrics: (v: boolean) => void;
   setShowChangelogOnUpdate: (v: boolean) => void;
   setLastSeenChangelogVersion: (v: string) => void;
+  setEnableHiRes: (v: boolean) => void;
   setHotCacheEnabled: (v: boolean) => void;
   setHotCacheMaxMb: (v: number) => void;
   setHotCacheDebounceSec: (v: number) => void;
@@ -142,6 +146,7 @@ export const useAuthStore = create<AuthState>()(
       showFullscreenLyrics: true,
       showChangelogOnUpdate: true,
       lastSeenChangelogVersion: '',
+      enableHiRes: false,
       hotCacheEnabled: false,
       hotCacheMaxMb: 256,
       hotCacheDebounceSec: 30,
@@ -222,6 +227,7 @@ export const useAuthStore = create<AuthState>()(
       setShowChangelogOnUpdate: (v) => set({ showChangelogOnUpdate: v }),
       setLastSeenChangelogVersion: (v) => set({ lastSeenChangelogVersion: v }),
 
+      setEnableHiRes: (v) => set({ enableHiRes: v }),
       setHotCacheEnabled: (v) => set({ hotCacheEnabled: v }),
       setHotCacheMaxMb: (v) => set({ hotCacheMaxMb: v }),
       setHotCacheDebounceSec: (v) => set({ hotCacheDebounceSec: v }),