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/getting-started/facility-automation.md

@@ -111,7 +111,7 @@ Create a dashboard and add a gauge bound to `house.living_temperature`.
 Publish the dashboard to see the current reading. Dashboard controls can also
 publish messages, so choose their bindings with care.
 
-See [Dashboards and panels](../interface/dashboards.md) for widgets and pairing
+See [Dashboards and devices](../interface/dashboards.md) for widgets and pairing
 a wall tablet.
 
 For longer-term history, add an **InfluxDB** node that consumes