feat(zigbee): add slow-fetch hint in modal + bump timeout to 300s

- Modal: small italic note under Test/Fetch buttons warning users that
  large meshes can take several minutes
- Service: _NETWORKMAP_TIMEOUT 180s -> 300s (5 min) for very large meshes

backend/app/services/zigbee_service.py

@@ -18,7 +18,7 @@ except ImportError:  # pragma: no cover
 _NETWORKMAP_REQUEST_TOPIC = "{base_topic}/bridge/request/networkmap"
 _NETWORKMAP_RESPONSE_TOPIC = "{base_topic}/bridge/response/networkmap"
 _CONNECTION_TIMEOUT = 5.0   # seconds to verify broker reachability
-_NETWORKMAP_TIMEOUT = 180.0  # seconds to wait for the networkmap response (large meshes can be slow)
+_NETWORKMAP_TIMEOUT = 300.0  # seconds to wait for the networkmap response (large meshes can be slow)
 
 
 def _sanitize_mqtt_error(exc: BaseException) -> str:

frontend/src/components/zigbee/ZigbeeImportModal.tsx

@@ -309,6 +309,9 @@ export function ZigbeeImportModal({ open, onClose, onAddToCanvas }: ZigbeeImport
                 Fetch Devices
               </Button>
             </div>
+            <p className="text-[11px] text-muted-foreground italic">
+              Fetching the network map can take several minutes on large meshes.
+            </p>
           </div>
 
           {/* Device List */}