Progress-ring image format for buttons

[sebastientaggart]

Context

Clock-face progress visual (thin ring for remaining, bold arc for completed) for usage buttons. Nice-to-have, not v1.

Scope

• Image generator (Pillow) producing 72×72 and 96×96 PNGs given {percent, label, color}.
• New Data Widget format: progress_ring.
• OpenDeck plugin pushes generated image via SetImage.

Out of scope

Stream Deck SDK port (separate effort).

Acceptance

• Button bound to a usage state key with progress_ring format renders a ring that updates live.

Depends on

#21 (needs a real usage state key to demo against).