feat(timer): add choice between pausing and stopping media on expiry

[Xitee1]

Summary

Adds a setting to choose what happens to media playback when the timer expires: Pause (the previous, still-default behavior) or Stop.

• New MediaEndAction enum (Pause/Stop) in core:data, persisted via DataStore as media_end_action, defaulting to Pause so existing users see no behavior change.
• MediaVolumeController.fadeOutAndPause → fadeOutAndEndPlayback(durationSeconds, endAction): after the fade-out it dispatches KEYCODE_MEDIA_PAUSE or KEYCODE_MEDIA_STOP depending on the setting. The fade logic itself is unchanged.
• Settings UI: a "Pause or stop" picker row directly under the Playback toggle, showing the current selection in the description slot and opening a two-option dialog — same pattern as the auto-rotate mode picker. Strings added in en and de.

Note on semantics: KEYCODE_MEDIA_STOP is delivered to the active media session like pause, but its interpretation is player-dependent — most apps end the session and dismiss their media notification; a few treat it identically to pause.

Testing

• ./gradlew assembleDebug — passes
• ./gradlew lint — passes

🤖 Generated with Claude Code

[Xitee1]

Doesn't work, in stop mode the music starts playing again just a after it has stopped/faded out