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/types.ts

@@ -73,8 +73,71 @@ export interface Device extends DeviceSummary {
   photo_count: number
   created_at: string
   updated_at: string
+  mhi_gateway_id?: number | null
+  mhi_unit_index?: number | null
+  mhi_iu_address?: number | null
+  mhi_register_map?: Record<string, MhiRegisterField> | null
 }
 
+// ── MHI Modbus gateways (manual aircon control) ──────────────────────────
+export interface MhiRegisterField {
+  address: number
+  active: boolean
+  readWrite: string | null
+}
+
+export interface MhiImportedUnit {
+  unitIndex: number
+  iu: number | null
+  ou: number | null
+  fields: Record<string, MhiRegisterField>
+}
+
+export interface MhiImportResult {
+  meta: {
+    projectName: string | null
+    mapsVersion: string | null
+    internalProtocol: string | null
+    externalProtocol: string | null
+    timestamp: string | null
+  }
+  global: Record<string, MhiRegisterField>
+  units: MhiImportedUnit[]
+  unmatchedRows: { row: number; description: string }[]
+}
+
+export interface MhiGateway {
+  id: number
+  name: string
+  host: string
+  port: number
+  slave_id: number
+  address_base: number
+  imported_filename: string | null
+  imported_units: MhiImportResult | null
+  imported_at: string | null
+  created_at: string
+  updated_at: string
+  assigned_count: number
+}
+
+export interface MhiStatus {
+  external_ref: string
+  onOff?: string
+  mode?: string
+  setpoint?: number
+  fanSpeed?: string
+  roomTemp?: number
+  errorCode?: number
+  compressor?: string
+  commStatus?: string
+  filterSign?: string
+  error?: string
+}
+
+export const MHI_MODES = ['cool', 'heat', 'fan', 'auto', 'dry'] as const
+export const MHI_FAN_SPEEDS = ['low', 'medium', 'high', 'powerful'] as const
+
 export interface DevicePhoto {
   id: number
   device_id: number