feat: Add dual subtitle playback with translation support.

[whyzgeek]

Dual subtitles with on-the-fly translation

Adds a "dual subtitles" mode that renders a second translated line below the
primary subtitle while a video plays. This is very useful for language learners. The translation source is picked
automatically:

1. YouTube's official tlang timedtext when the selected caption track is
   marked translatable — same XML the YouTube site uses, no extra latency, and
   line breaks stay aligned with the primary track.
2. Google Translate web endpoint as a per-line fallback for tracks that
   can't be translated by YouTube (e.g. user-uploaded SRT-style captions),
   with an LRU cache of the last 200 lines.
   The translated line is rendered in italic light-green directly under the
   original by a small extension to the SmartTube ExoPlayer fork's
   SubtitlePainter, so both lines paint as a single block (no flicker, no
   parallel SubtitleView overlays).

Why a Painter-level change

The translated text is merged into the same Cue as the original, then tagged
with an android.text.Annotation span (smarttube-dual / translation).
SubtitlePainter detects the span and renders the second line with a separate
TextPaint, so the merge:

• doesn't fight applyEmbeddedStyles (the user's caption-style prefs still
  apply to the primary line),
• preserves cue background colors on both lines,
• keeps a single positioning origin (no second view to keep in sync with
  player resize, AFR switching, PiP, etc.).

User-facing controls

• Subtitles → Dual subtitles — single toggle.
• Subtitles → Secondary translated subtitle — language picker (20 common
  languages).
• The same language picker is also reachable from the in-player subtitle
  dialog and shows a warning if dual subtitles isn't enabled yet.

Settings persistence

Three new fields appended to the existing PlayerData serialization
(indices 63, 65, 66) — backwards-compatible with existing saved prefs.

[whyzgeek]

I prepped this using AI and tested all worked on Android device and FireTV Stick