fix(music): show the endless radio switch as on while a started station plays

[AmyJeanes]

Summary

A station started with Start radio keeps playing even when endless radio is off, but the queue sheet's switch only showed the saved setting: it read off while the station ran, and turning it off didn't stop the station.

• EnhancedMusicPlayerManager.kt: the explicit-station flag moves out of Media3MusicService into a session-only radioStationActive flow (reset with the queue), so the UI can see it. The service sets it on an explicit reseed and reads it when extending.
• MusicRadioSwitch.kt (new, split out of MusicPlayerViewModel like MusicPlayerLyrics): the switch shows on for the saved setting or a running station, so one tap stops whichever is running; turning it off ends the station too.
• EnhancedMusicPlayerManager.playTrack ends a named station. The service reads a new queue that shares a track with the station as the same session (MusicRadioPlanner.resolveQueueContext), so the station used to carry on, unseen, into the next queue the user played. Start radio still re-marks it from its pending seed.
• Tests: MusicRadioSwitchTest (4) and MusicRadioStationTest (1).

Follows the desktop behaviour agreed in Flow-Tube/Flow-Desktop#93.

Related issue

None filed. Follow-up to Flow-Tube/Flow-Desktop#93, as mentioned there.

Change type

• Bug fix
• Feature
• Refactor or maintenance
• Build, packaging, or CI
• Documentation

Validation

• ./gradlew :app:assembleGithubDebug
• ./gradlew :app:testGithubDebugUnitTest (3141 tests, 0 failures)
• I ran any additional flavor-specific build or test tasks affected by this change. (compileFossDebugKotlin, spotlessCheck, ktlintCheck)
• I manually tested the affected behavior on an Android device or emulator.

Test device and Android version:

Pixel 10 Pro · Android 17 (SDK 37)

Endless radio off: Start radio shows the switch on and keeps refilling; one tap turns it off and nothing more is added. Starting a station, then playing a song that was already in it, ends the station. With the setting on, turning the switch off clears both.

Screenshots or recordings

start-radio-switch-android.mp4

Risk and compatibility

Session-only state; the saved setting and its migration are untouched. The only playback change is that starting a new queue ends a named station, matching desktop.

AI disclosure: This PR was largely written by Claude (Opus 5.5) but was fully reviewed and tested by myself before raising this.

• The change does not introduce secrets, private data, or unexpected telemetry.
• New user-facing text uses Android string resources. (no new text)
• Dependency and lockfile changes are intentional and limited to this PR. (none)
• Room schema changes include the required version bump and migration, or this PR does not change the Room schema.
• Breaking changes and upgrade steps are clearly documented. (none)