Add audio-only music mode

Persistent per-platform mode that plays only the audio track to save
bandwidth and speed up loads. When enabled, stream selection picks the
best audio-only stream (preferred language, codec, bitrate) and never
fetches the video URL; toggling mid-video reloads at the current
position using the quality-switch path (live vid=no toggling is avoided
due to A/V desync).

- Toggle row in the player quality/settings sheet on iOS, macOS and
  tvOS, next to lock controls
- New audioMode control button type (red when active) for player
  controls and mini player layouts
- Picking a video quality explicitly turns the mode off; audio track
  picks keep it on and now work while audio-only is playing
- Stream URL refresh preserves audio-only playback instead of
  resurrecting video
- Queue preloads and history entries resolved before a toggle are
  discarded so selection re-runs with the current mode
- tvOS player shows the video thumbnail instead of a black screen
  during audio-only playback
- PiP button hidden for audio-only streams

Yattee/Localizable.xcstrings

@@ -1389,6 +1389,16 @@
         }
       }
     },
+    "controls.button.audioMode" : {
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Audio Mode"
+          }
+        }
+      }
+    },
     "controls.button.autoPlayNext" : {
       "localizations" : {
         "en" : {
@@ -7687,6 +7697,17 @@
         }
       }
     },
+    "player.quality.audioMode" : {
+      "comment" : "Audio-only (music) mode toggle in the player settings sheet",
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Audio Mode"
+          }
+        }
+      }
+    },
     "player.quality.loading" : {
       "comment" : "Loading streams message in quality selector",
       "localizations" : {