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/components/panels/__tests__/Sidebar.test.tsx

@@ -70,6 +70,7 @@ const defaultProps = {
   onAddText: vi.fn(),
   onScan: vi.fn(),
   onZigbeeImport: vi.fn(),
+  onZwaveImport: vi.fn(),
   onSave: vi.fn(),
   onOpenSettings: vi.fn(),
   onOpenHistory: vi.fn(),
@@ -184,6 +185,12 @@ describe('Sidebar', () => {
     expect(defaultProps.onAddGroupRect).toHaveBeenCalledOnce()
   })
 
+  it('calls onZwaveImport when Z-Wave Import is clicked', () => {
+    render(<Sidebar {...defaultProps} />)
+    fireEvent.click(screen.getByText('Z-Wave Import'))
+    expect(defaultProps.onZwaveImport).toHaveBeenCalledOnce()
+  })
+
   it('calls onSave when Save Canvas is clicked', () => {
     render(<Sidebar {...defaultProps} />)
     fireEvent.click(screen.getByText('Save Canvas'))