Fix listening post agent mode and UI sync

Agent scanner fixes:
- Use non-blocking I/O with select/fcntl to prevent blocking reads
- Pass dwell_time parameter through to scanner function
- Add freqs_scanned counter to status and data endpoints
- Improve SDR test process cleanup with kill() fallback

Frontend listening post fixes:
- Add setListeningPostRunning for UI sync when switching to agent
- Fix button ID (radioScanBtn not scannerStartBtn)
- Handle nested data structure from controller proxy
- Update freqs_scanned and signal_count from polling data
- Disable listen button for agent mode (audio can't stream over HTTP)

Add listening_post to agents.js uiSetters map for mode sync.

Live testing completed:
- Sensor mode: works via agent
- WiFi quick scan: works via agent
- Listening post: works via agent (AM airband, WFM broadcast tested)
- Signal detection: confirmed working via agent

Testing ongoing - modes not yet tested via agent:
- Pager, ADS-B, AIS, ACARS, APRS, DSC, RTL-AMR, TSCM, Bluetooth

static/js/core/agents.js

@@ -679,7 +679,8 @@ function syncModeUI(mode, isRunning, agentId = null) {
         'adsb': 'setADSBRunning',
         'wifi': 'setWiFiRunning',
         'bluetooth': 'setBluetoothRunning',
-        'acars': 'setAcarsRunning'
+        'acars': 'setAcarsRunning',
+        'listening_post': 'setListeningPostRunning'
     };
 
     const setterName = uiSetters[mode];