[global] power: add native battery alerts and Hub settings

[Sipper1236]

Low-battery alerts now run as a native shell service across every bar style. Win+, → Graphics & Power → Battery exposes a master enable switch, independent warning and critical switches, editable percentages, and a Test notification action. Users can choose warning-only, critical-only, both, or neither; both levels default to enabled. The Test message reflects which levels are enabled. Defaults are 25% and 10%; the daemon validates and persists the settings, including requiring critical below warning.

The service sends “Battery low” and “Battery critical” through the normal notification system, respects Do Not Disturb, and emits each level once per discharge session. AC reconnect resets the session; startup below critical sends only the critical alert. Successful delivery consumes an alert, while failures remain eligible for retry. The existing battery indicator needs no extra widget. Python's separate low/critical notifications are replaced by this service; its full-charge notification remains.

The Battery settings card is available even without hardware charge-limit controls. A dedicated native IPC target avoids collisions with the community add-on. Users with that add-on should disable its alerts when adopting the built-in service to avoid duplicate reminders.

Validation:

• Shell daemon: all 566 tests pass; go vet and go build pass.
• Native offscreen QML simulation verifies default/custom thresholds, critical startup, charger reset, disabled alerts, deduplication, Test delivery, and failed delivery retry, warning-only, critical-only, both-disabled, and the matching Test messages; notify-send is mocked.
• Isolated Hub control probe verifies input bounds and setting/RPC acknowledgment handling.
• Every Hub section loads in the existing offscreen page probe.
• Full shell and Hub QML lint, delivery, compositor isolation, git hooks, and diff checks pass.

No installed desktop files or live settings were changed. This PR targets unstable-dev; the Ryostore community add-on remains a separate submission in Ryoku-dev/ryostore#73.

[neur0map]

Thanks for the work here. Reworking this into the next v1 release.