feat: add band_extract audio source type for frequency band filtering

New audio source type that filters a parent source to a specific frequency
band (bass 20-250Hz, mid 250-4kHz, treble 4k-20kHz, or custom range).
Supports chaining with frequency range intersection and cycle detection.
Band filtering applied in both CSS audio streams and test WebSocket.

server/src/wled_controller/static/js/app.ts

@@ -148,7 +148,7 @@ import {
     showAudioSourceModal, closeAudioSourceModal, saveAudioSource,
     editAudioSource, cloneAudioSource, deleteAudioSource,
     testAudioSource, closeTestAudioSourceModal,
-    refreshAudioDevices,
+    refreshAudioDevices, onBandPresetChange,
 } from './features/audio-sources.ts';
 
 // Layer 5: value sources
@@ -474,6 +474,7 @@ Object.assign(window, {
     testAudioSource,
     closeTestAudioSourceModal,
     refreshAudioDevices,
+    onBandPresetChange,
 
     // value sources
     showValueSourceModal,