feat(mcp): add design_id to approve_device tool

The backend approve endpoint already accepts design_id via NodeCreate,
but the MCP schema didn't expose the field so callers couldn't target
a specific canvas when approving a discovered device. Add _DESIGN_ID_FIELD
to approve_device so AI clients (and the MCP bridge) can pass it through.

mcp/app/tools.py

@@ -115,6 +115,7 @@ def _build_tools() -> list[Tool]:
                 "id":    {"type": "string"},
                 "type":  {"type": "string", "enum": NODE_TYPES, "default": "generic"},
                 "label": {"type": "string"},
+                **_DESIGN_ID_FIELD,
             },
         }),
         Tool(name="hide_device", description="Hide a pending discovered device", inputSchema={