Panels become devices: per-device documents in the flow repo, a Devices tab

- flow/devices.py: DeviceDef in FLOWS_DIR/_devices/<id>.json, committed under
  the flow store's write lock, 409 on a stale version; nonces in DEVICES_FILE
  outside git; in-memory presence announced as device_status
- api/routes/devices.py: CRUD, pairing with an optional {name, board,
  firmware}, a pending list adopted without a code from the LAN only, unpair;
  /panels/pair, /panels/pair/{code} and GET /panels/{id} kept as aliases,
  PUT /panels/ gone
- _device_may replaces _panel_may, JWT claims unchanged
- panels.json moves into devices once at startup, then .migrated
- MCP list_devices, get_device; search category device
- Devices tab (list, detail with presence, pairing, ordered dashboards,
  touch, I/O), PanelsDialog removed, serialised optimistic saves

docs/code/cli.md

@@ -366,7 +366,7 @@ directory. Common choices:
 | `FLUKSIO_HOME` | Select the CLI's data directory |
 | `NODE_VENV` | `auto` adopts an existing venv; `managed` requests a separate one; a path selects an interpreter |
 | `REDIS_HOST` | Use Redis for flow state and queues |
-| `FRONTEND_HOST` | Browser address used in mail, OAuth metadata and panel links |
+| `FRONTEND_HOST` | Browser address used in mail, OAuth metadata and device links |
 | `ENVIRONMENT` | `production` disables interactive API documentation |
 | `MCP_ENABLED` | Enable the [agent endpoint](agents.md) |