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>

internal/server/server.go

@@ -60,8 +60,9 @@ type RouteMounter interface {
 // /api/v1/integrations/google and its /connect, /callback and /calendars,
 // PUT /api/v1/integrations/google/calendar and DELETE
 // /api/v1/integrations/google; pushSvc serves GET
-// /api/v1/push/vapid-public-key and POST and DELETE
-// /api/v1/push/subscriptions; runner serves
+// /api/v1/push/vapid-public-key, POST and DELETE
+// /api/v1/push/subscriptions and POST
+// /api/v1/settings/notifications/test-push; runner serves
 // POST /api/v1/jobs and GET /api/v1/jobs/{id}; photos serves
 // GET /media/photos/{id}-{size}.jpg; assets is the web build
 // served for every other GET. /health, /auth and the web build are public.
@@ -133,6 +134,7 @@ func NewRouter(logger *slog.Logger, version string, trusted forwarded.Trusted, a
 	apiV1.Patch("/settings/household", patchHouseholdSettings(logger, settings))
 	apiV1.Get("/settings/notifications", getNotificationSettings(logger, settings))
 	apiV1.Put("/settings/notifications", putNotificationSettings(logger, settings))
+	apiV1.Post("/settings/notifications/test-push", postTestPush(logger, pushSvc))
 	apiV1.Get("/settings/recipe-search", getRecipeSearchSettings(logger, search))
 	apiV1.Patch("/settings/recipe-search", patchRecipeSearchSettings(logger, search))
 	apiV1.Post("/settings/recipe-search/test", postRecipeSearchTest(logger, search))