fix: Initialize audio visualizer when listening starts

The audio visualizer (Web Audio API analyzer) was not being initialized
when direct listening or scanner signal detection started, so the
synthesizer never received audio level data.

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

static/js/modes/listening-post.js

@@ -659,6 +659,8 @@ function handleSignalFound(data) {
                 scannerAudio.volume = knobValue / 100;
             }
             scannerAudio.play().catch(e => console.warn('[SCANNER] Audio autoplay blocked:', e));
+            // Initialize audio visualizer to feed signal levels to synthesizer
+            initAudioVisualizer();
         }
     }
 
@@ -2089,6 +2091,9 @@ async function _startDirectListenInternal() {
             console.log('[LISTEN] Initial play blocked, waiting for canplay');
         });
 
+        // Initialize audio visualizer to feed signal levels to synthesizer
+        initAudioVisualizer();
+
         isDirectListening = true;
         updateDirectListenUI(true, freq);
         addScannerLogEntry(`${freq.toFixed(3)} MHz (${currentModulation.toUpperCase()})`, '', 'signal');