Add "Only Forced Subtitles" option to subtitle selection (#589)

## Description

Adds a client-side "Only Forced Subtitles" option when selecting
subtitles, per the discussion in #571.

  ### Changes

  - **TrackIndex.ONLY_FORCED** (`-3`): New constant for the special case
- **StreamChoiceService**: Intercepts `ONLY_FORCED` at the top of
`chooseSubtitleStream()` before server-side `SubtitlePlaybackMode` logic
(which remains untouched)
  - **UI**: Adds "Only Forced Subtitles" option to both:
    - In-player subtitle menu (`PlaybackDialog.kt`)
    - Pre-playback stream selection dialog (`Dialogs.kt`)

  ### Forced track detection

Uses metadata `isForced` flag as primary check, with title-based
fallback ("forced", "signs", "songs" patterns) when language matches
audio.

  ### Related issues

  Closes #571

  ### Screenshots

  N/A - subtitle selection menu only

  ### AI/LLM usage

  Used Claude Code for assistance and to draft PR description.

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Damontecres <damontecres@gmail.com>

app/src/main/res/values/strings.xml

@@ -75,6 +75,7 @@
     <string name="no_scheduled_recordings">No scheduled recordings</string>
     <string name="no_update_available">No update available</string>
     <string name="none">None</string>
+    <string name="only_forced_subtitles">Only Forced Subtitles</string>
     <string name="outro">Outro</string>
     <string name="path">Path</string>
     <string name="people">People</string>