Turn on push from onboarding step 3 and add a test-push endpoint (Feature 19)

Onboarding step 3's "Turn on notifications" now calls usePush().subscribePush
straight from that tap (the only place onboarding asks for permission) and
shows how it ended: on; blocked, with how to allow it in the browser's site
settings; not supported, with how to add MealQuest to the Home Screen on
iPhone; or a dismissed prompt that can be asked again. "Maybe later" still
finishes without asking. The interim notificationsUnavailable is gone.

POST /api/v1/settings/notifications/test-push queues "MealQuest test
notification" through the notification outbox as a plan_ready event with a
fresh plan_ready:test-<ULID> key, opening /plan, on the push channel only
(new notify.Service.EmitPush), never Google Chat. It answers 202
{event_key, subscriptions}, or 409 no_subscriptions / push_off rather than
reporting success when nothing would be sent.

Maestro task f89279b8-b92c-4d9f-a6c2-fd87fccda509.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -706,6 +706,17 @@ Runs are recorded in `schedule_runs` and executed through `jobs`.
     `{"endpoint"}` and answers 204 whether or not it was stored.
   - An invalid body is 400 `invalid_request` (not a JSON object of the
     known fields) or `invalid_subscription` (a field fails validation).
+  - `POST /api/v1/settings/notifications/test-push` (CSRF-checked) checks
+    delivery to the household's devices. It queues "MealQuest test
+    notification" through `notification_outbox`, so it takes the real
+    delivery path with its retries. It is a `plan_ready` event whose tap
+    opens `/plan`. Its key is `plan_ready:test-<ULID>`, new on every call,
+    so `Emit` never deduplicates it. It goes to the `push` channel only
+    (`notify.Service.EmitPush`), never to the family Google Chat space.
+    It answers 202 `{event_key, subscriptions}`. Two cases send nothing
+    and answer 409 rather than report success: no device has subscribed
+    (`no_subscriptions`), or push is switched off for `plan_ready` in
+    the preferences (`push_off`).
 - `internal/chat` keeps the Google Chat incoming webhook in `chat_settings`
   and is the outbox's sender for the `chat` channel. The webhook URL holds
   its own key and token, so it is sealed with `secretbox` in