Add controls for SoundTouch input sources

[jelledelaender]

Summary

Add Homey controls that let users switch a SoundTouch speaker input source, starting with common inputs such as AUX and Bluetooth.

Context

The app currently focuses on local preset events and internet radio stream playback through UPnP AVTransport. Users also need a way to change the speaker source from Homey, for example when switching from a managed radio preset to AUX or Bluetooth.

Scope

• Investigate the SoundTouch local API command needed to switch sources such as AUX and BLUETOOTH / BT.
• Add one or more Homey controls or Flow actions for selecting an input source.
• Keep the feature local-only and compatible with older Homey Pro runtime constraints.
• Avoid disrupting the existing active preset capability state; switching away from preset/radio playback should clear or update active preset state consistently.
• Surface clear user-facing errors when a speaker or model does not support a requested source.

Acceptance criteria

• A user can switch a paired SoundTouch speaker to AUX from Homey.
• A user can switch a paired SoundTouch speaker to Bluetooth from Homey where supported.
• Unsupported or failed source changes produce a useful warning/error instead of silently failing.
• Existing preset playback and active preset state behavior still works after switching inputs.
• Relevant tests or manual verification notes cover the new source switching behavior.

Implementation notes

Likely areas to inspect:

• lib/soundtouch-client.js for SoundTouch API helpers.
• drivers/soundtouch/device.js for device actions and active preset state handling.
• drivers/soundtouch/driver.js for Flow action registration.
• Compose/manifest files if adding new Flow actions or capabilities.