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/utils/nodeIcons.ts

@@ -11,7 +11,7 @@ import {
   // Security & Auth
   Shield, ShieldCheck, Lock, Key, Users, UserCheck, Flame,
   // Automation & IoT
-  Zap, Workflow, Bot, Home, Thermometer, Lightbulb, Radio, BotMessageSquare, Webhook,
+  Zap, Workflow, Bot, Home, Thermometer, Lightbulb, Radio, RadioTower, Share2, BotMessageSquare, Webhook,
   // Smart Home / Sensors
   Plug, Power, BatteryCharging, Sun, DoorOpen, KeyRound, AlarmSmoke, Siren,
   Radar, PersonStanding, Vibrate, Droplet, Droplets, Wind, AirVent, Fan,
@@ -178,6 +178,9 @@ export const NODE_TYPE_DEFAULT_ICONS: Record<NodeType, LucideIcon> = {
   zigbee_coordinator: Radio,
   zigbee_router:      Zap,
   zigbee_enddevice:   Lightbulb,
+  zwave_coordinator:  RadioTower,
+  zwave_router:       Share2,
+  zwave_enddevice:    Lightbulb,
   generic:           Circle,
   group:             Circle,
   groupRect:         Circle,