docs: add multi-node master dashboard documentation

- Architecture overview with diagram
- Enabling master mode (is_master config)
- Node selector UI indicators
- Switching between local and peer views
- Live tracking on remote nodes
- Proxy API endpoints for peer data
- Peer registration examples
- WebSocket peer connection details
- Troubleshooting section for common issues

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

USAGE.md

@@ -281,6 +281,131 @@ The web server exposes a REST API:
 | POST | `/api/autoscan/stop` | Stop auto-scanning |
 | GET | `/api/bluetooth/identify/<addr>` | Identify BT device |
 
+## Multi-Node Master Dashboard
+
+RF Mapper supports a multi-node architecture where one designated "master" node can view and control scanning on peer nodes without page redirects.
+
+### Architecture Overview
+
+```
+┌─────────────────────────────────────────────────────────┐
+│  Master Node (rpios)                                     │
+│  ┌───────────────────────────────────────────────────┐  │
+│  │  Node Selector: [📍 rpios ▼]                      │  │
+│  │                  ├─ 📍 rpios (local)              │  │
+│  │                  ├─ 📡 grokbox                    │  │
+│  │                  └─ 📡 jellystar                  │  │
+│  └───────────────────────────────────────────────────┘  │
+│           │                │                │           │
+│       [Radar]          [3D Map]      [Device Lists]     │
+└─────────────────────────────────────────────────────────┘
+                         │
+        ┌────────────────┼────────────────┐
+        ▼                ▼                ▼
+    grokbox:5000    jellystar:5000    (local)
+```
+
+### Enabling Master Mode
+
+Set `is_master: true` in `config.yaml` on the master node:
+
+```yaml
+scanner:
+  id: 'rpios'
+  name: 'Master Scanner'
+  is_master: true          # Enable master dashboard features
+  peers: []                # Peers auto-register via API
+  accept_registrations: true
+```
+
+Peer nodes do NOT need this flag (defaults to `false`).
+
+### Node Selector
+
+When `is_master: true` and peers are registered, a node selector dropdown appears in the header:
+
+| Indicator | Meaning |
+|-----------|---------|
+| 📍 | Local scanner (this node) |
+| 📡 | Peer scanner (remote node) |
+| ● (green) | Connected and responding |
+| ● (yellow) | Connecting/loading |
+| ○ (red) | Peer unreachable |
+
+### Switching Between Nodes
+
+1. Click the node selector dropdown
+2. Select a peer node (e.g., "📡 grokbox")
+3. Dashboard recenters on peer's GPS location
+4. Device lists show peer's scanned devices
+5. WebSocket connects to peer for real-time updates
+
+When viewing a peer:
+- Map centers on peer's coordinates
+- Radar shows devices relative to peer
+- "New Scan" triggers scan on peer node
+- Live tracking runs scans on peer
+
+### Live Tracking on Remote Nodes
+
+Live tracking works across nodes:
+
+1. Select a peer from node selector
+2. Enable "Live BT Track" toggle
+3. Scans run on the selected peer (not locally)
+4. Results stream back via WebSocket
+5. Map updates in real-time with peer's devices
+
+### Proxy API Endpoints
+
+The master node provides proxy endpoints to access peer data:
+
+| Endpoint | Description |
+|----------|-------------|
+| `/api/node/<id>/latest` | Get latest scan from peer |
+| `/api/node/<id>/scan/bt` | Trigger BT scan on peer |
+| `/api/node/<id>/device/floors` | Get device floor assignments |
+| `/api/node/<id>/positions/trilaterated` | Get trilaterated positions |
+
+Example:
+```bash
+# Get grokbox's latest scan via master
+curl http://rpios:5000/api/node/grokbox/latest | jq
+
+# Trigger BT scan on grokbox
+curl -X POST http://rpios:5000/api/node/grokbox/scan/bt
+```
+
+### Peer Registration
+
+Peers register with the master automatically or manually:
+
+```bash
+# Manual registration from peer
+curl -X POST http://rpios:5000/api/peers/register \
+  -H "Content-Type: application/json" \
+  -d '{
+    "id": "grokbox",
+    "name": "Grokbox Scanner",
+    "url": "http://grokbox:5000",
+    "latitude": 50.858495,
+    "longitude": 4.397614,
+    "floor": 11
+  }'
+
+# List registered peers
+curl http://rpios:5000/api/peers | jq '.peers'
+```
+
+### WebSocket Peer Connections
+
+When viewing a peer node, the dashboard:
+
+1. Disconnects from local WebSocket
+2. Connects to peer's WebSocket at `peer_url/ws/scan`
+3. Receives real-time scan updates from peer
+4. Falls back to HTTP polling if WebSocket fails
+
 ## Data Storage
 
 Scan results are saved as JSON in the data directory:
@@ -333,3 +458,21 @@ adb shell "settings put global settings_enable_monitor_phantom_procs false"
 1. Disable battery optimization for Termux in Android settings
 2. Run `termux-wake-lock` before starting RF Mapper
 3. The app acquires wake lock automatically on start
+
+### Master Dashboard: Node selector not appearing
+1. Verify `is_master: true` in config.yaml
+2. Restart rf-mapper: `rf-mapper restart`
+3. Check peers are registered: `curl http://localhost:5000/api/peers | jq '.peers | length'`
+4. At least one peer must be registered for selector to appear
+
+### Master Dashboard: Peer shows red status
+1. Check peer is running: `curl http://peer:5000/api/peers`
+2. Check network connectivity: `ping peer`
+3. Check firewall allows port 5000
+4. Re-register peer if needed (see Peer Registration above)
+
+### Master Dashboard: WebSocket to peer fails
+1. Check peer WebSocket endpoint: `curl http://peer:5000/socket.io/`
+2. Browser console shows connection errors
+3. Falls back to HTTP polling automatically
+4. Ensure peer is running with WebSocket support enabled