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.

docs/USER_GUIDE.md

@@ -182,6 +182,9 @@ The **Proton account** section on this page is where you sign in. See
 **Playback**
 
 - **Audio language** and **Subtitles:** the language to start in, or the file's own default.
+- **Subtitle size**, **Subtitle background** and **Subtitle colour:** how plain subtitles (SRT)
+  look. Choose **Dark box** if text is hard to read on bright scenes. Styled subtitles (ASS) keep
+  their own look and only change size. These apply from the next file.
 - **Play the next episode** (on by default) and **Skip openings and credits** (off).
 - **Hardware decoding** (on). Turn it off if the picture shows green blocks or stays black.