Add per-TRV control panel + valve/wifi telemetry, restyle dashboard radiator icon

Backend:
- New valve_position column, populated from shellies/<id>/info alongside
  wifi_rssi (was already a column, never actually written to).
- New GET/POST /api/devices/:id/trv-status and /trv-control routes, mirroring
  the mhi-status/mhi-control and daikin-status/daikin-control pattern. Control
  goes through the existing setTargetWithRetry() reliability pipeline.

Frontend:
- New TrvControlPanel.tsx: +/- setpoint control debounced 1s after the last
  click before firing a command (these are battery/deep-sleep valves going
  through a real retry/backoff pipeline, not a cheap register write like MHI/
  Daikin), plus room temp, valve position %, wifi signal, and a live "time
  since last update" readout. Wired into ZoneDetailModal for shelly_trv rows.
- ZoneCard dashboard tile: radiator icon now just shows red when heating_active
  (valve open) / grey when idle, with only the target setpoint alongside it
  (dropped the room-temp half, redundant with the zone-level temp line above).

frontend/src/types.ts

@@ -93,6 +93,7 @@ export interface DeviceSummary {
   // via MQTT (mqtt.js); mhi_modbus populates the rest via the AC poller (ac-poller.js).
   current_room_temp: string | null
   heating_active: boolean | null
+  valve_position: number | null
   power_state: 'on' | 'off' | null
   current_mode: string | null
   fan_speed: string | null
@@ -189,6 +190,20 @@ export interface DaikinStatus {
   error?: string
 }
 
+export interface TrvStatus {
+  external_ref: string
+  setpoint?: number
+  roomTemp?: number
+  heatingActive?: boolean | null
+  valvePosition?: number | null
+  wifiRssi?: number | null
+  batteryPct?: number | null
+  targetOrigin?: string | null
+  lastSeen?: string | null
+  statusUpdatedAt?: string | null
+  error?: string
+}
+
 export interface DevicePhoto {
   id: number
   device_id: number