feat(ring): optionally open the ring screen over other apps

A full-screen intent is only a heads-up on a phone in use. A new, opt-in
setting under Alarms lets a ringing alarm or timer open its screen over
the app in use instead. It relies on "display over other apps", which
turning the setting on sends the user to grant; the switch reads as off
while the permission is missing. A locked or dark phone is still left
to the full-screen intent, so the screen never opens twice.

docs/ARCHITECTURE.md

@@ -996,6 +996,7 @@ declared because a specific path would otherwise fail, silently, at 07:00.
 | `WAKE_LOCK` | keep the CPU up for the length of a ring |
 | `VIBRATE` | an alarm and a timer both vibrate, and vibrate alone when no audio source opens |
 | `ACCESS_NOTIFICATION_POLICY` | issue #1's opt-in bedtime mode. Install-time, and on its own it does nothing: it lists Clockula under Special app access › Do Not Disturb, which the user grants by hand. With that grant Clockula adds one Do Not Disturb rule of its own, which always lets alarms through (§18) |
+| `SYSTEM_ALERT_WINDOW` | the opt-in "Ring screen over other apps". Granted by hand from the system page the setting opens. It is used only as Android's background-activity-start exemption, so a ring screen opens over the app in use where a full-screen intent would be a heads-up. Clockula draws no overlay window |
 
 **M6 adds no permission at all.** A timer never takes over the screen (no
 full-screen intent, no ring activity, no `showWhenLocked`), because its user set