feat(tv): add native Android TV support

[PastaHimself]

Summary

• add automatic TV detection with Automatic, Mobile, and TV overrides
• add a Leanback launcher entry while keeping the existing application ID and APK
• add a remote-friendly TV navigation root for Home, Subscriptions, Search, Library, Settings, and playback
• reuse Flow's existing repositories, shared player state, Media3 manager, downloads, and data stores
• keep phone and tablet navigation on the existing mobile path
• add focused unit coverage for mode resolution, TV destinations, and remote media-key mapping

Architecture

MainActivity remains Flow's single lifecycle and playback host. It reacts to the stored interface mode and renders either the existing FlowApp or the new FlowTvApp. TvActivity is only a Leanback launcher bridge.

The interface preference is stored in its own Preferences DataStore, so this change does not touch the Room schema or require a migration.

Validation

• static XML and whitespace checks passed locally
• local Gradle execution was unavailable because the sandbox could not resolve the Gradle distribution host
• GitHub Actions is the authoritative compile and test gate for this branch

Manual checks still needed

• launch from an Android TV or Google TV home screen
• verify D-pad focus traversal across the rail, rows, search, library, and player controls
• verify media keys and back handling during playback
• verify switching Automatic/Mobile/TV preserves active playback
• verify existing phone launch, navigation, PiP, and deep links