Device: partial e-ink refresh for ticking widgets

[fberrez]

Use the panel's partial refresh for frequently-changing regions (focus timer, clock-with-seconds) instead of a full-flash refresh every tick.

Scope (device/)

• Detect/declare which region(s) update often (server can hint via a header or render metadata).
• Drive partial refresh on the Waveshare panel for those regions; periodic full refresh to clear ghosting.

Why

• The prototype's partial refresh is why its Pomodoro didn't flicker the whole screen. Needed before ticking widgets feel good on real hardware. Relates to Device ↔ server pairing: WiFi provisioning, QR claim/approve, studio auth + multi-tenant-ready schema #1 (device work).