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/index.md

@@ -13,7 +13,8 @@ The sidebar opens as a sheet on phones.
 |---|---|
 | **Home** | see the brain graph, health and recent activity |
 | **Flows** | create flows and open their editors |
-| **Dashboards** | build widget dashboards and pair wall panels |
+| **Dashboards** | build widget dashboards |
+| **Devices** | pair wall screens and choose the dashboards each shows |
 | **Runs** | start batch runs and review their results |
 | **Workers** | manage machines that execute nodes |
 | **Secrets** | store credentials; superusers only |
@@ -29,7 +30,7 @@ the docs in another tab.
 ### Search
 
 Use **Search**, or ⌘K / Ctrl-K, to find flows, nodes, flow inputs and outputs,
-dashboards, widgets, panels, secrets, modules, workers and alert channels.
+dashboards, widgets, devices, secrets, modules, workers and alert channels.
 Selecting a result opens its editor or settings.
 
 Search covers the current instance. When connected through a portal,
@@ -91,6 +92,6 @@ Light and dark follow your system by default. Change this under
 ## Next stops
 
 - [The flow editor](flow-editor.md)
-- [Dashboards and panels](dashboards.md)
+- [Dashboards and devices](dashboards.md)
 - [Secrets, modules and alerts](operations.md)
 - [Accounts and the portal](portal.md)