feat: add multi-node master dashboard

Enable a designated "master" node to view device data from any peer node
within the same dashboard, without page redirects.

- Add is_master config option to ScannerConfig
- Add proxy endpoints /api/node/<id>/* for peer data
- Add node selector dropdown UI (shown only on master)
- Add peer WebSocket connection support for real-time updates
- Live tracking uses node-specific scan endpoint
- Map centers on selected peer's position

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

TODO.md

@@ -37,6 +37,7 @@
 - [x] WebSocket real-time sync (instead of polling)
 - [ ] Automatic peer discovery via mDNS/Bonjour
 - [x] Sync RSSI history for trilateration
+- [x] Master dashboard: view peer node data without redirect
 - [ ] Web UI for peer management
 - [ ] Sync conflict resolution UI
 
@@ -248,6 +249,7 @@
 - [x] Termux/Android environment detection with prerequisite checks
 - [x] Multi-scanner trilateration for device positioning
 - [x] Signal coverage heat map visualization
+- [x] Multi-node master dashboard (view peer data in single UI)
 
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