Add real-time audio spectrum test for audio sources and templates

- Add WebSocket endpoints for live audio spectrum streaming at ~20Hz
- Audio source test: resolves device/channel, shares stream via ref-counting
- Audio template test: includes device picker dropdown for selecting input
- Canvas-based 64-band spectrum visualizer with falling peaks and beat flash
- Channel-aware: mono sources show left/right/mixed spectrum correctly

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -48,6 +48,7 @@ import {
     showTestTemplateModal, closeTestTemplateModal, onEngineChange, runTemplateTest,
     showAddAudioTemplateModal, editAudioTemplate, closeAudioTemplateModal, saveAudioTemplate, deleteAudioTemplate,
     cloneAudioTemplate, onAudioEngineChange,
+    showTestAudioTemplateModal, closeTestAudioTemplateModal, startAudioTemplateTest,
     showAddStreamModal, editStream, closeStreamModal, saveStream, deleteStream,
     onStreamTypeChange, onStreamDisplaySelected, onTestDisplaySelected,
     showTestStreamModal, closeTestStreamModal, updateStreamTestDuration, runStreamTest,
@@ -111,6 +112,7 @@ import {
 import {
     showAudioSourceModal, closeAudioSourceModal, saveAudioSource,
     editAudioSource, cloneAudioSource, deleteAudioSource,
+    testAudioSource, closeTestAudioSourceModal,
 } from './features/audio-sources.js';
 
 // Layer 5: value sources
@@ -247,6 +249,9 @@ Object.assign(window, {
     deleteAudioTemplate,
     cloneAudioTemplate,
     onAudioEngineChange,
+    showTestAudioTemplateModal,
+    closeTestAudioTemplateModal,
+    startAudioTemplateTest,
 
     // kc-targets
     createKCTargetCard,
@@ -343,6 +348,8 @@ Object.assign(window, {
     editAudioSource,
     cloneAudioSource,
     deleteAudioSource,
+    testAudioSource,
+    closeTestAudioSourceModal,
 
     // value sources
     showValueSourceModal,