feat(offline): make every dashboard write durable offline (P2–P5)

Builds on the outbox engine to take the whole dashboard offline in one place
instead of wiring 114 mutation sites individually.

Frontend (single chokepoint = the API client):
- offline-write: any write auto-queues to the outbox on offline/network failure
  and returns an optimistic value; the online path is unchanged apart from an
  Idempotency-Key header (so even online retries de-dup). entityType is derived
  from the URL; POSTs get a remappable local id.
- client.doWrite unifies POST/PUT/PATCH/DELETE through this path. WriteOptions
  gains `offline: "queue" | "reject" | "manual"`.
- Guardrails: auth / billing / payments / SMS / exports are online-only and throw
  OFFLINE_UNAVAILABLE offline rather than queueing (no queued double-charges or
  surprise SMS blasts). use-api-error resolves the friendly localized message
  (fa/en/ar).
- submit-order opts out ("manual") to keep its richer local-Order mock; shared
  helpers de-duplicated into offline-write.
- Request persistent storage on mount so unsynced writes survive eviction.

Backend:
- IdempotencyCleanupJob: daily purge of idempotency records older than 7 days
  (the table now gets a row per keyed write). Registered in Hangfire. No migration.

86 API tests pass; dashboard tsc + build clean.

web/dashboard/messages/ar.json

@@ -25,7 +25,8 @@
     "notFound": "غير موجود",
     "unauthorized": "غير مصرح",
     "network": "خطأ في الاتصال",
-    "generic": "حدث خطأ. حاول مرة أخرى."
+    "generic": "حدث خطأ. حاول مرة أخرى.",
+    "OFFLINE_UNAVAILABLE": "يتطلب هذا الإجراء اتصالاً بالإنترنت. يرجى المحاولة بعد عودة الاتصال."
   },
   "brand": {
     "name": "ميزي"