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>

cmd/server/main.go

@@ -160,7 +160,7 @@ func run(cfg config.Config, logger *slog.Logger) error {
 		return err
 	}
 
-	pushSvc, err := push.NewService(db, cfg.VAPID.PublicKey)
+	pushSvc, err := push.NewService(db, cfg.VAPID.PublicKey, notifier)
 	if err != nil {
 		return err
 	}