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/push.go

@@ -5,6 +5,7 @@ import (
 	"log/slog"
 	"net/http"
 
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/notify"
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/push"
 )
 
@@ -85,6 +86,32 @@ func deletePushSubscription(logger *slog.Logger, svc *push.Service) http.Handler
 	}
 }
 
+// postTestPush answers POST /settings/notifications/test-push: it queues
+// push.TestTitle for every device that turned on push, aimed at /plan, and
+// answers 202 with the event key and how many subscriptions it goes to.
+// Delivery follows through the notification outbox. With no subscription
+// it is 409 no_subscriptions, and with push switched off for plan_ready
+// 409 push_off; neither sends anything. It never posts to Google Chat.
+func postTestPush(logger *slog.Logger, svc *push.Service) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		sent, err := svc.SendTest(r.Context())
+		switch {
+		case errors.Is(err, push.ErrNoSubscriptions):
+			writeError(w, r, logger, http.StatusConflict, "no_subscriptions",
+				"no device has turned on notifications, so there is nothing to send the test to")
+		case errors.Is(err, notify.ErrPushOff):
+			writeError(w, r, logger, http.StatusConflict, "push_off",
+				"push is switched off for New weekly plan ready in Notifications settings, so no test was sent")
+		case err != nil:
+			logger.ErrorContext(r.Context(), "send test push", "error", err)
+			writeError(w, r, logger, http.StatusInternalServerError, "internal", "internal error")
+		default:
+			logger.InfoContext(r.Context(), "test push queued", "event_key", sent.EventKey, "subscriptions", sent.Subscriptions)
+			writeJSON(w, r, logger, http.StatusAccepted, sent)
+		}
+	}
+}
+
 // writePushError maps a push.Service error to its API error: an invalid
 // subscription is 400 invalid_subscription, anything else 500.
 func writePushError(w http.ResponseWriter, r *http.Request, logger *slog.Logger, action string, err error) {