v0.0.2 — Battery uptime tracking

**Battery uptime display**
The battery screen now shows time on battery in the bottom-right corner (e.g. "On battery: 4h 32m" or "On battery: 1d 2h 15m"). On first boot, the WiFi QR screen silently fetches the current time via NTP and stores it in deep-sleep-safe memory. The timer resets on power cycle — which is exactly when you'd want it to. Subsequent loads of the QR screen are fully offline.

**README**
Added a README covering hardware requirements, modes, setup, and how the deep sleep architecture works. Written for public release.

**Weather screen polish**
Minor position adjustment to the high temperature label.

Auto-refresh weather every 4 hours via TimeAlarm

When in weather mode (mode 1), sleep with a 4-hour TimeAlarm alongside
the pin alarms. On TimeAlarm wake, sleep_memory[0] is still 1 so the
weather screen re-fetches and re-renders automatically.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>