Add sleep timer to player bar (#1671)

* feat: add sleep timer to player bar

- Add sleep timer button in player bar right controls
- Preset options: End of song, 5/10/15/30/45 min, 1 hr, 2 hrs
- Custom timer with HH:MM:SS input fields
- Timer only counts down while music is playing
- Timer pauses playback when it expires
- End-of-song mode pauses at the next track change
- Uses theme-aware styling (--theme-colors-surface)
- Add sleepTimer/sleepTimerOff icons (LuTimer/LuTimerOff)
- Add i18n strings for sleep timer UI

---------

Co-authored-by: York <york@BonecharMac.local>
Co-authored-by: jeffvli <jeffvictorli@gmail.com>

src/i18n/locales/en.json

@@ -667,7 +667,16 @@
         "trackRadio": "track radio",
         "unfavorite": "unfavorite",
         "pause": "pause",
-        "viewQueue": "view queue"
+        "viewQueue": "view queue",
+        "sleepTimer": "sleep timer",
+        "sleepTimer_endOfSong": "end of current song",
+        "sleepTimer_minutes": "{{count}} min",
+        "sleepTimer_hours": "{{count}} hr",
+        "sleepTimer_custom": "custom",
+        "sleepTimer_off": "off",
+        "sleepTimer_timeRemaining": "{{time}} remaining",
+        "sleepTimer_setCustom": "set timer",
+        "sleepTimer_cancel": "cancel timer"
     },
     "queryBuilder": {
         "standardTags": "standard tags",