feat(core,player,app): subtitle size, background and colour settings

Plain-text subtitles (SRT) were drawn in mpv's bare default, thin white
text that bright scenes swallow. Settings → Playback now picks a size,
an outline or a dark box, and white or yellow text. Styled (ASS) tracks
keep their own look and take only the size.

The box options are mpv 0.38's, so they are set best effort: an older
libmpv logs and plays on. The style syncs with the other playback
settings, and the Android bridge keeps the stored style instead of
resetting it on save.

crates/pstr-android/src/lib.rs

@@ -1909,7 +1909,11 @@ impl AndroidEngine {
     pub fn set_playback_prefs(&self, prefs: PlaybackPrefsRecord) -> Result<(), BridgeError> {
         // Sanitized rather than trusted: the volume arrives from a slider, and
         // a language from a text field that can hold spaces and nothing else.
-        let prefs = playback_prefs(prefs).sanitized();
+        // Onto what is stored, not the defaults: the record has no subtitle
+        // style yet, and saving one without it would reset the desktop's and,
+        // through sync, every other installation's.
+        let stored = pstr_core::prefs::load(&self.dirs).unwrap_or_default();
+        let prefs = playback_prefs(prefs, stored).sanitized();
         pstr_core::prefs::save(&self.dirs, &prefs).map_err(BridgeError::from_display)
     }
 
@@ -3580,7 +3584,7 @@ fn playback_prefs_record(prefs: PlaybackPrefs) -> PlaybackPrefsRecord {
     }
 }
 
-fn playback_prefs(record: PlaybackPrefsRecord) -> PlaybackPrefs {
+fn playback_prefs(record: PlaybackPrefsRecord, stored: PlaybackPrefs) -> PlaybackPrefs {
     PlaybackPrefs {
         volume: record.volume,
         muted: record.muted,
@@ -3590,6 +3594,7 @@ fn playback_prefs(record: PlaybackPrefsRecord) -> PlaybackPrefs {
         autoplay_next: record.autoplay_next,
         auto_skip: record.auto_skip,
         speed: record.speed,
+        ..stored
     }
 }