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).

backend/src/db.js

@@ -183,6 +183,10 @@ export async function initDb() {
   // back in mqtt.js's status handler. A mismatch = guest changed it via the dial.
   await pool.query(`ALTER TABLE zone_devices ADD COLUMN IF NOT EXISTS last_commanded_target_t NUMERIC(4,1)`)
 
+  // Valve position (0-100, from thermostats[0].pos on shellies/<id>/info) — >0 already
+  // drives heating_active, but the modal wants the raw percentage too.
+  await pool.query(`ALTER TABLE zone_devices ADD COLUMN IF NOT EXISTS valve_position INT`)
+
   // Default-template rollout: existing installs created occupied_temp/vacant_temp/
   // heating_offset_min/cooling_offset_min as NOT NULL with a hardcoded default — relax
   // that so NULL can mean "inherit the global hvac_defaults template" (see seedDefaults()