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/components/CustomSelect.tsx

@@ -15,9 +15,10 @@ interface Props {
   onChange: (value: string) => void;
   className?: string;
   style?: React.CSSProperties;
+  disabled?: boolean;
 }
 
-export default function CustomSelect({ value, options, onChange, className = '', style }: Props) {
+export default function CustomSelect({ value, options, onChange, className = '', style, disabled }: Props) {
   const [open, setOpen] = useState(false);
   const triggerRef = useRef<HTMLButtonElement>(null);
   const listRef = useRef<HTMLDivElement>(null);
@@ -80,7 +81,8 @@ export default function CustomSelect({ value, options, onChange, className = '',
         type="button"
         className={`custom-select-trigger ${className}`}
         style={style}
-        onClick={() => setOpen(v => !v)}
+        disabled={disabled}
+        onClick={() => { if (!disabled) setOpen(v => !v); }}
         aria-haspopup="listbox"
         aria-expanded={open}
       >