Add global occupied/vacant default templates with per-room overrides

Settings gains a 'Default Templates' section: arrival-prep/departure-prep offsets,
a default aircon template (on/off, mode, setpoint, fan speed x occupied/vacant), and
default TRV setpoints. Every room can individually override any of these fields via
per-field 'Override' toggles in its Schedule modal, falling back to the global
default when not overridden.

Backend:
- zones table: occupied_temp/vacant_temp/heating_offset_min/cooling_offset_min are
  now nullable (NULL = inherit default); existing values preserved as explicit
  per-room overrides, so no room's behaviour changes on deploy. 8 new nullable
  ac_occupied_*/ac_vacant_* override columns.
- New 'hvac_defaults' config key (generic config store) holds the global template.
- scheduler.js: resolves each field as zone-override ?? global-default, and now
  also drives AC state automatically on occupancy transitions (previously AC was
  100% manual) via the same command path the manual control panel uses.
- lib/ac-control.js: factored the mhi_mqtt/mhi_modbus command logic out of
  routes/override.js so the scheduler and the manual control route share one path.
- zones.js PATCH: explicit-null-clears-override semantics for the 12 overridable
  fields (distinct from omitting the key, which leaves the value unchanged).

Frontend:
- Settings.tsx: new Default Templates section matching the requested layout.
- ZoneDetailModal.tsx: per-field Override checkboxes for offsets, AC
  occupied/vacant template, and TRV occupied/vacant setpoints.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/routes/override.js

@@ -1,6 +1,7 @@
 import { requireAuth, requireCap } from '../auth.js'
 import { pool, logActivity } from '../db.js'
-import { batchSetZoneTemperature, publishMhiCommand } from '../lib/mqtt.js'
+import { batchSetZoneTemperature } from '../lib/mqtt.js'
+import { commandAcDevice } from '../lib/ac-control.js'
 import * as mhiDriver from '../lib/drivers/mhi-modbus.js'
 
 // Manual force-temperature — the equivalent of the old integration's
@@ -69,12 +70,11 @@ export async function overrideRoutes(app) {
     return status
   })
 
-  // POST /api/devices/:id/mhi-control — manual aircon control only (no scheduler
-  // wiring). Body: { onOff?, mode?, tempC?, fanSpeed? } — only the fields present
-  // are written. mhi_mqtt: publish command topics (fire-and-forget, QoS 1); the
-  // gateway echoes the new state back on its status topics within a second, which
-  // mqtt.js re-caches — we also optimistically update the cache for instant UI.
-  // mhi_modbus: synchronous register write via the driver.
+  // POST /api/devices/:id/mhi-control — explicit staff control from the room modal's
+  // MhiControlPanel. Body: { onOff?, mode?, tempC?, fanSpeed? } — only the fields present
+  // are written. The actual write path (mhi_mqtt publish vs mhi_modbus register write,
+  // plus optimistic cache update) lives in lib/ac-control.js's commandAcDevice(), shared
+  // with the scheduler's automatic occupied/vacant AC control (lib/scheduler.js).
   app.post('/api/devices/:id/mhi-control', { preHandler: requireCap('control') }, async (req, reply) => {
     const { rows } = await pool.query(
       `SELECT * FROM zone_devices WHERE id = $1 AND device_type IN ('mhi_modbus', 'mhi_mqtt')`,
@@ -84,43 +84,7 @@ export async function overrideRoutes(app) {
     const device = rows[0]
     const { onOff, mode, tempC, fanSpeed } = req.body || {}
 
-    let ok
-    if (device.device_type === 'mhi_mqtt') {
-      try {
-        publishMhiCommand(device.external_ref, { onOff, mode, tempC, fanSpeed })
-        ok = true
-      } catch (err) {
-        ok = false
-        req.log.warn(`MHI command publish failed for ${device.external_ref}: ${err.message}`)
-      }
-      if (ok) {
-        // Optimistic cache update so the dashboard reflects the command immediately.
-        await pool.query(
-          `UPDATE zone_devices SET
-             power_state         = COALESCE($1, power_state),
-             current_mode        = COALESCE($2, current_mode),
-             current_target_temp = COALESCE($3, current_target_temp),
-             fan_speed           = COALESCE($4, fan_speed),
-             updated_at = NOW()
-           WHERE id = $5`,
-          [
-            onOff === undefined || onOff === null ? null : (onOff ? 'on' : 'off'),
-            mode ?? null,
-            tempC ?? null,
-            fanSpeed ?? null,
-            device.id,
-          ]
-        )
-      }
-    } else {
-      ok = await mhiDriver.setTarget(device.external_ref, { onOff, mode, tempC, fanSpeed })
-      if (ok && tempC !== undefined && tempC !== null) {
-        await pool.query(
-          `UPDATE zone_devices SET current_target_temp = $1, target_origin = 'manual', updated_at = NOW() WHERE id = $2`,
-          [tempC, device.id]
-        )
-      }
-    }
+    const { ok } = await commandAcDevice(device, { onOff, mode, tempC, fanSpeed }, { origin: 'manual', log: req.log })
 
     await logActivity(device.zone_id, 'device_command', {
       note: `MHI manual control on ${device.location || device.discovered_name || device.external_ref}: ${JSON.stringify({ onOff, mode, tempC, fanSpeed })}${ok ? '' : ' — FAILED'}`,