feat(alarms): swipe to delete, with Undo

An alarm card swipes away towards the end edge, or deletes from its
"Delete" accessibility action, and the list's snackbar offers Undo.
The editor's Delete drops its confirmation for the same Undo, routed
through a singleton so the list can answer it after the editor pops.
Timer cards get the same Undo for their corner delete.

AlarmEngine.delete stops a ringing alarm and clears a snoozed one's
notification under the engine lock; restore puts the row back under
its own id with its ring state and reschedules. A ringing alarm comes
back dismissed, never ringing; a finished timer comes back reset.

The swipe is hand-rolled so the end-to-start drag stays with the tab
pager.

Fixes #20

docs/ROADMAP.md

@@ -247,7 +247,8 @@ line ("in 9h 12m") on each row. Built on `GroupedSurface`/`GroupedRow`.
 
   Knowingly open: no hand reordering of alarms (the time order answers it, and
   `alarms` has no `sort_order` column), no multi-select or bulk delete, no undo
-  chip after a delete (traded for a confirmation that names the alarm), no
+  chip after a delete (traded for a confirmation that names the alarm; since
+  replaced by swipe to delete with Undo, #20), no
   repeat presets, and no per-alarm auto-silence (still M10 at the earliest).
   Editing the app-wide defaults every override row offers to follow is M10's
   settings screen. The seven new instrumentation tests compile in the gate but
@@ -316,7 +317,7 @@ Elapsed-realtime anchored.
   per-timer vibrate, volume-ramp or dismiss-challenge overrides (`timers` has
   one nullable settings column and four more would be a schema change), no
   "stop all" when several timers are expired, no undo after a delete (traded for
-  a confirmation, as with alarms), no ramp on a timer's ring, no presets
+  a confirmation, as with alarms; added since, #20), no ramp on a timer's ring, no presets
   management screen and no editing of `default_timer_duration_millis` or
   `default_timer_ringtone_uri` (M10's settings screen). `TimerSnapshot.anchorIsStale`
   is still not surfaced in the UI. Of `AlarmClock`'s intent contract only the