Port the Mac app's scene system to the phone control. Each queue entry is
now an independent clone carrying a `cues` list of timed elements (effect /
text / image), each with its own appearance time and visible duration; the
three channels are driven independently from playback progress (advanceCues).
Add visual-only "Intermezzo" queue items (silence audio, run their scene for
a set duration) and make the inter-track gap keep the scene running. New
mobile scene editor lives in the queue card detail (⚙). The queue (with
scenes) is persisted via saveState so it survives a phone reload.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
