feat(flow): retry failed orders

Add ability to re-import individual ERROR/SKIPPED orders directly from
the order detail modal. Downloads narrow date range from GoMag API,
finds the specific order, and re-runs import_single_order().

Backend:
- New retry_service.py with retry_single_order() — downloads order_date
  ±1 day from GoMag, finds order by number, imports via import_service
- Guard: blocks retry during active sync (_sync_lock check)
- POST /api/orders/{order_number}/retry endpoint

Frontend:
- "Reimporta" button in modal footer (visible only for ERROR/SKIPPED)
- Spinner during retry, success/error feedback with auto-refresh

Cache-bust: shared.js?v=18

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/routers/sync.py

@@ -484,6 +484,16 @@ async def order_detail(order_number: str):
     return detail
 
 
+@router.post("/api/orders/{order_number}/retry")
+async def retry_order(order_number: str):
+    """Retry importing a failed/skipped order."""
+    from ..services import retry_service
+
+    app_settings = await sqlite_service.get_app_settings()
+    result = await retry_service.retry_single_order(order_number, app_settings)
+    return result
+
+
 @router.get("/api/dashboard/orders")
 async def dashboard_orders(page: int = 1, per_page: int = 50,
                            search: str = "", status: str = "all",