feat: initial commit — local services (backend + manager dashboard + waiter PWA)

Includes all work to date:
- local_backend: FastAPI backend with products, orders, tables, shifts, cloud sync
- manager_dashboard: React manager UI with product/category management, reports, settings
- waiter_pwa: React PWA for waiter devices
- Category reparent endpoint and UI
- Waiter domain: local_ip sent on heartbeat, waiter_domain persisted from cloud response
- QR code modal in AppInfoTab for waiter domain
- Product form: number input spinners removed, category pre-selected on new product
- Category row: count badge moved to far right

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

local_backend/schemas/base.py

@@ -0,0 +1,14 @@
+from datetime import datetime
+from typing import Annotated
+from pydantic import PlainSerializer
+
+# SQLite strips tzinfo on read-back, so naive datetimes from DB are actually UTC.
+# This serializer appends "Z" so browsers parse them correctly as UTC.
+UTCDatetime = Annotated[
+    datetime,
+    PlainSerializer(
+        lambda dt: (dt.isoformat() + "Z") if dt.tzinfo is None else dt.isoformat(),
+        return_type=str,
+        when_used="json",
+    ),
+]