Add Flow triggers for speaker playback changes

[jelledelaender]

Goal

Add Homey Flow triggers for useful SoundTouch state changes.

Candidate Triggers

• Started playing.
• Stopped playing.
• Preset changed.
• Source changed.
• Speaker became unavailable.

Context

The app already listens to Bose websocket updates and syncs speaker_playing, active preset, source, and availability. Flow triggers would let users automate around those changes.

Acceptance Criteria

• Trigger cards are available for playback and preset/source changes.
• Triggers fire only on actual state transitions, not every status refresh.
• Useful tokens are included where appropriate, e.g. preset number/name or source.
• npx homey app validate passes.