feat(audio): notify user when selected output device goes missing (issue #307)

Level A of issue #307. When the configured output device has
disappeared from the host device list at the time of play/resume
(KVM switched away, USB sink unplugged, PipeWire sink removed…)
the backend emits an 'audio:device-missing' event and the frontend
shows a warning toast:

  'Output device "XYZ" is no longer available. Using system default.'

The existing init_device fallback already routes audio to the system
default, so this is purely informational — the user now understands
why audio came out of a different sink instead of staring at a silent
progress bar.

Does NOT touch the sacred audio init path (pipewire_backend.rs,
init_device, audio_settings.rs). New module audio_device_watch.rs
adds a pure presence check, a new-state throttle so we don't emit on
every resume once the user's been notified, and a v2 command
v2_check_audio_device_presence so the frontend can poll on demand.

Ready for Level B (background watchdog) — the same helpers plug into
a timer without further refactor; Level B only needs to add a tokio
task that calls emit_missing_if_needed() every N seconds while
playback is active.

cc

src/lib/i18n/locales/de.json

@@ -1687,7 +1687,8 @@
     },
     "laodingDevices": "Geräte werden geladen...",
     "findingSimilarArtists": "Suche nach ähnlichen Künstlern...",
-    "awardUnavailable": "Detailansicht für diese Auszeichnung nicht verfügbar."
+    "awardUnavailable": "Detailansicht für diese Auszeichnung nicht verfügbar.",
+    "audioDeviceMissing": "Ausgabegerät '{device}' ist nicht mehr verfügbar. Standardgerät wird verwendet."
   },
   "offline": {
     "title": "Offline-Modus",