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/locales/en.json

@@ -791,6 +791,19 @@
   "audio_source.error.name_required": "Please enter a name",
   "audio_source.audio_template": "Audio Template:",
   "audio_source.audio_template.hint": "Audio capture template that defines which engine and settings to use for this device",
+  "audio_source.test": "Test",
+  "audio_source.test.title": "Test Audio Source",
+  "audio_source.test.rms": "RMS",
+  "audio_source.test.peak": "Peak",
+  "audio_source.test.beat": "Beat",
+  "audio_source.test.connecting": "Connecting...",
+  "audio_source.test.error": "Audio test failed",
+
+  "audio_template.test": "Test",
+  "audio_template.test.title": "Test Audio Template",
+  "audio_template.test.device": "Audio Device:",
+  "audio_template.test.device.hint": "Select which audio device to capture from during the test",
+  "audio_template.test.run": "🧪 Run",
 
   "audio_template.title": "🎵 Audio Templates",
   "audio_template.add": "Add Audio Template",