Add MHI aircon manual control (Phase 2, Modbus only)

Manual on/off/mode/setpoint/fan control for MHI indoor units via the
already-verified Intesis Modbus TCP gateway, deliberately NOT wired into
the NewBook-driven scheduler yet — that stays deferred. Modbus doesn't
depend on the MQTT broker (separate infra, still unbuilt), so this can be
deployed and tested standalone.

- mhi_gateways table (connection config: host/port/slave id/address base)
  + zone_devices columns for MHI devices (gateway id, unit index, IU hint,
  and the per-device register map staged from a MAPS import — ground
  truth, never re-derived from room/IU at runtime)
- drivers/mhi-modbus.js: discover/getStatus/setTarget over modbus-serial,
  same interface as trv.js/homeassistant.js; per-gateway request queue
  since Modbus TCP requires serialised requests; falls back to the
  profile's stride formula (with a loud warning) only if a unit has never
  been through an import
- routes/mhi-gateways.js: gateway CRUD, live test-connection, xlsx import
  (stages the parsed result for review — never auto-creates devices),
  per-unit 'assign to zone' as the explicit commit step
- routes/override.js: GET .../mhi-status, POST .../mhi-control (control cap)
- frontend: GatewaysPanel (add gateway, test connection, import + preview,
  assign units) and MhiControlPanel (on/off, mode, setpoint, fan) wired
  into Devices.tsx / ZoneDetailModal.tsx

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

frontend/src/api.ts

@@ -1,5 +1,6 @@
 import type {
   Zone, ZoneStatus, Device, DevicePhoto, ActivityEntry, NewbookSite, AppConfig,
+  MhiGateway, MhiStatus,
 } from './types'
 
 const BASE = '/hvac/api'
@@ -96,6 +97,41 @@ export function photoUrl(filePath: string): string {
   return `${BASE}/uploads${filePath}`
 }
 
+// MHI Modbus gateways (manual aircon control)
+export function fetchMhiGateways(): Promise<MhiGateway[]> {
+  return request('/mhi-gateways')
+}
+export function createMhiGateway(body: { name: string; host: string; port?: number; slaveId?: number; addressBase?: number }): Promise<MhiGateway> {
+  return request('/mhi-gateways', { method: 'POST', body: JSON.stringify(body) })
+}
+export function updateMhiGateway(id: number, body: Partial<{ name: string; host: string; port: number; slaveId: number; addressBase: number }>): Promise<MhiGateway> {
+  return request(`/mhi-gateways/${id}`, { method: 'PATCH', body: JSON.stringify(body) })
+}
+export function testMhiGatewayConnection(id: number): Promise<{ ok: boolean; note?: string; error?: string }> {
+  return request(`/mhi-gateways/${id}/test-connection`, { method: 'POST', body: JSON.stringify({}) })
+}
+export async function importMhiRegisterMap(id: number, file: File): Promise<MhiGateway> {
+  const form = new FormData()
+  form.append('file', file)
+  const res = await fetch(`${BASE}/mhi-gateways/${id}/import`, { method: 'POST', credentials: 'include', body: form })
+  if (!res.ok) {
+    const err = await res.json().catch(() => ({ error: res.statusText }))
+    throw new Error(err.error || `Import failed: ${res.status}`)
+  }
+  return res.json()
+}
+export function assignMhiUnit(gatewayId: number, unitIndex: number, body: { zone_id: number | null; location: string }): Promise<Device> {
+  return request(`/mhi-gateways/${gatewayId}/units/${unitIndex}/assign`, { method: 'POST', body: JSON.stringify(body) })
+}
+
+// MHI manual control (device-level — mhi_modbus rows only)
+export function fetchMhiStatus(deviceId: number): Promise<MhiStatus> {
+  return request(`/devices/${deviceId}/mhi-status`)
+}
+export function sendMhiControl(deviceId: number, body: { onOff?: boolean; mode?: string; tempC?: number; fanSpeed?: string }): Promise<{ ok: boolean }> {
+  return request(`/devices/${deviceId}/mhi-control`, { method: 'POST', body: JSON.stringify(body) })
+}
+
 // Config / settings
 export function fetchConfig(): Promise<AppConfig> {
   return request('/config')