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/index.css

@@ -227,8 +227,8 @@ html, body, #root { height: 100%; margin: 0; font-size: 14px; }
 
 .rad-row { display: flex; gap: 10px; margin-top: 8px; flex-wrap: wrap; }
 .rad-icon { display: flex; align-items: center; gap: 3px; font-size: 10.5px; font-weight: 600; color: var(--text-mid); }
-.rad-icon.heating { color: var(--st-heating-up); }
-.rad-icon.idle     { color: var(--st-cooling-down); }
+.rad-icon.heating { color: var(--danger); }
+.rad-icon.idle     { color: var(--text-mid); }
 
 .ac-block { position: relative; display: flex; flex-direction: column; align-items: center; gap: 2px; padding-top: 4px; color: var(--text-dark); }
 .ac-block.off { color: var(--text-mid); opacity: .55; }