gnome: adaptive refresh cadence parity with CLI and macOS menubar

The scoped remainder of #389, whose Lifetime half was fixed by #753 and #759.

The CLI dashboard and the macOS menubar both run the adaptive refresh cadence (activity-aware interval, nap-aware backstop); the GNOME extension still refreshes on its original fixed schedule. Bring it to parity:

• adopt the same interval policy the macOS menubar uses (fast while sessions are active, slow when idle)
• keep the existing GNOME battery/idle behavior intact
• document the cadence in the extension README

Needs a Linux/GNOME environment to test, which is why it did not move with the rest of #389; contributions welcome, a maintainer can validate behavior descriptions against the macOS implementation.