feat: pause polling while locked or suspended - #3590

Summary

Part of OpenSpec change 04-clean-up-dependencies-and-electron-debt (tasks 3.1–3.5 and 6.1).

• Emit a new SYSTEM_SLEEP event from powerMonitor suspend and lock-screen; keep the existing SYSTEM_WAKE events on resume and unlock-screen.
• Add a session-local usePowerStateStore and a usePowerState subscription mounted in GlobalEffects.
• Gate notification and account refetchIntervals on the power state.
• Wake/unlock resumes polling with an immediate refetch; duplicate wake events use refetch({ cancelRefetch: false }) so they share the in-flight fetch.
• Document the session power state and polling behavior in ARCHITECTURE.md.

Polling behavior

• Hidden or unfocused window: polling continues; refetchIntervalInBackground keeps the timers running.
• Offline: notification and account queries pause (networkMode: 'online'), and the notifications screen shows the offline state; on reconnect they resume, and the online state is re-probed on wake.
• Locked or suspended: SYSTEM_SLEEP disables the query intervals; SYSTEM_WAKE re-enables them and triggers an immediate refetch of the notification and account queries (cancelRefetch: false) before the normal intervals resume.

Verification

• pnpm check passes.
• Targeted tests pass: main power events (12), preload (14), power store/hook, GlobalEffects (9), account polling, and notification polling (71 across the affected renderer files).

Notes

Draft for review. Based on main; #3587 has merged, and #3588 still touches the same events/preload/system-handler area, so expect a small conflict if #3588 lands first.