Add settings page, PIN manager, and auto-config

- New settings page at /settings (localhost-only) accessible via tray icon
- PIN manager with 6-digit codes, 5-min TTL, single-use
- API token uses secrets.token_urlsafe(32) for 256-bit entropy
- Config auto-creates config.yaml on first run with detected local IP
- Default PIN and token generated randomly on first startup
- Added pin_ttl setting (default 300s)
- Updated build_exe.py for onefile exe with settings assets bundled
- Added start.bat and start.sh for easy development startup

server/.env.example

@@ -8,6 +8,6 @@ DASHBOARD_PIN=1234
 API_TOKEN=your-secret-token-here
 
 # Server settings
-HOST=0.0.0.0
+# HOST defaults to auto-detected local IP (set to override)
 PORT=8921
 LOG_LEVEL=INFO