fix(mpv): auto-select subtitle and audio tracks on file open (#12)

Closes #7

mpv only turns on a subtitle that is flagged default or matches `slang`, so releases like VisionQuest (Russian dub by default, ru/ru/en subtitles, none flagged) open with subtitles off, which also disabled **Translate subtitle**.

- New `TrackSelection` helpers in `qtpotato_core` with unit tests.
- On file load: audio follows *Preferred languages* (Playback settings); subtitle follows *Preferred languages* (Subtitles settings), else keeps mpv's choice, else (new *Turn on a subtitle when none matches*, on by default) picks a full text subtitle in the system language, then English, then the audio language, then the default/first track.
- Translate is enabled whenever the file has a text subtitle; with none or a bitmap subtitle shown it translates the best text track (English first).

Verified in the real app on VisionQuest S01E01: subtitle 3 (en) is selected on open; with audio set to `en` and auto-select off, audio switches to track 7, subtitle stays off and Translate stays enabled with the English source.

Reviewed-on: #12
Co-authored-by: Tri Vo Khac <git@trivk.dev>

src/qml/Actions.qml

@@ -389,7 +389,7 @@ Item {
     }
 
     property Action translateSubtitle: Action {
-        enabled: root.shortcutsEnabled && root.hasFile && root.mpv.subtitleId > 0 && !root.translator.busy
+        enabled: root.shortcutsEnabled && root.hasFile && root.mpv.subtitleTracks.some(track => !track.bitmap) && !root.translator.busy
         text: KI18n.i18nc("@action", "Translate Subtitle into %1", QtPotatoSettings.targetLanguage)
         icon.name: "translate"
         shortcut: "Ctrl+T"