feat: add Z-Wave network scan via MQTT gateway

Import a Z-Wave JS UI (zwavejs2mqtt) network over the MQTT gateway API,
mirroring the existing Zigbee pipeline:

- New Z-Wave Import modal + sidebar entry (broker, prefix, gateway name)
- coordinator/router/end-device typing with mesh tree from node neighbors
- import to Pending section or straight to canvas
- Pending Devices gains a Z-Wave source filter
- shared mqtt_common helpers extracted from the zigbee service

ha-relevant: yes

frontend/src/api/__tests__/client.test.ts

@@ -217,4 +217,14 @@ describe('api/client', () => {
     mod.zigbeeApi.importToPending(cfg)
     expect(api.post).toHaveBeenCalledWith('/zigbee/import-pending', cfg)
   })
+
+  it('zwaveApi.testConnection/importNetwork/importToPending', () => {
+    const cfg = { mqtt_host: 'h', mqtt_port: 1883, prefix: 'zwave', gateway_name: 'zwavejs2mqtt' }
+    mod.zwaveApi.testConnection(cfg)
+    expect(api.post).toHaveBeenCalledWith('/zwave/test-connection', cfg)
+    mod.zwaveApi.importNetwork(cfg)
+    expect(api.post).toHaveBeenCalledWith('/zwave/import', cfg)
+    mod.zwaveApi.importToPending(cfg)
+    expect(api.post).toHaveBeenCalledWith('/zwave/import-pending', cfg)
+  })
 })