gnome: adaptive refresh cadence parity with the menubar

[notlakshitha]

Summary

• The GNOME panel no longer polls on a fixed timer. It uses the macOS menubar intervals: 30s while the menu is open or a live session is still working, 120s when idle on AC, 150s on battery, and 300s in power saver.
• The Refresh Interval setting still applies. It caps the active poll (a shorter value is kept) and is kept when it is already slower than the idle floor.
• Opening the menu fetches immediately. The 5-minute payload cache still covers period and provider switches. A machine that does not answer UPower stays on the AC intervals, and the panel keeps polling while the session is locked. The macOS App Nap backstop (180s, 60s tolerance) is not armed, because GNOME Shell does not nap the extension.
• The cadence is documented in gnome/README.md.

Closes #892

Testing / Verification

• I have tested this locally against real data (not just unit tests)
• npm test passes
• `npm run build succeeds
• Not a UI change
• UI change: before and after screenshots attached below

npm test: 5203 tests passed, 24 skipped. npx tsc --noEmit passed. npm run build (CLI bundle and dashboard) succeeded. The new cadence cases are in tests/gnome-refresh-cadence.test.ts`. I have not run it in a live GNOME Shell session yet, so a check against the macOS behaviour would be appreciated.