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/interface/flow-editor.md

@@ -13,13 +13,13 @@ below, so a large flow grows downwards in one column.
 - **Edges** are routed around the nodes, a loop back to an earlier node
   included, and pulse when values pass through. Click one to inspect its
   payload.
-- **Endpoints** show flow inputs, dashboard widgets, panel I/O ports and nodes
+- **Endpoints** show flow inputs, dashboard widgets, device I/O ports and nodes
   in other flows that exchange messages with this flow. A value passing lights
   one up in a sweep: left to right when it sends into the flow, right to left
   when it reads out of it.
 - **Clicking an endpoint** shows a flow input's type, starting value and
-  current value, with a pencil that opens the flow settings to edit it. A panel
-  port opens its panel in the Panels dialog. A node in another flow opens that
+  current value, with a pencil that opens the flow settings to edit it. A device
+  port opens the device in the Devices tab. A node in another flow opens that
   flow on the label leading back here, with its edges highlighted.
 
 Values update over a websocket. A connection banner tells you when the editor