Add audio capture engine template system with multi-backend support

Introduces an engine+template abstraction for audio capture, mirroring the
existing screen capture engine pattern. This enables multiple audio backends
(WASAPI for Windows, sounddevice for cross-platform) with per-source
engine configuration via reusable templates.

Backend:
- AudioCaptureEngine ABC with WasapiEngine and SounddeviceEngine implementations
- AudioEngineRegistry for engine discovery and factory creation
- AudioAnalyzer class decouples FFT/RMS/beat analysis from engine-specific capture
- ManagedAudioStream wraps engine stream + analyzer in background thread
- AudioCaptureTemplate model and AudioTemplateStore with JSON CRUD
- AudioCaptureManager keyed by (engine_type, device_index, is_loopback)
- Auto-migration: default template created on startup, assigned to existing sources
- Full REST API: CRUD for audio templates + engine listing with availability flags
- audio_template_id added to MultichannelAudioSource model and API schemas

Frontend:
- Audio template cards in Streams > Audio tab with engine badge and config details
- Audio template editor modal with engine selector and dynamic config fields
- Audio template dropdown in multichannel audio source editor
- Template name crosslink badge on multichannel audio source cards
- Confirm modal z-index fix (always stacks above editor modals)
- i18n keys for EN and RU

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

server/src/wled_controller/main.py

@@ -24,6 +24,8 @@ from wled_controller.storage.picture_source_store import PictureSourceStore
 from wled_controller.storage.picture_target_store import PictureTargetStore
 from wled_controller.storage.color_strip_store import ColorStripStore
 from wled_controller.storage.audio_source_store import AudioSourceStore
+from wled_controller.storage.audio_template_store import AudioTemplateStore
+import wled_controller.core.audio  # noqa: F401 — trigger engine auto-registration
 from wled_controller.storage.value_source_store import ValueSourceStore
 from wled_controller.storage.profile_store import ProfileStore
 from wled_controller.core.profiles.profile_engine import ProfileEngine
@@ -45,11 +47,14 @@ picture_target_store = PictureTargetStore(config.storage.picture_targets_file)
 pattern_template_store = PatternTemplateStore(config.storage.pattern_templates_file)
 color_strip_store = ColorStripStore(config.storage.color_strip_sources_file)
 audio_source_store = AudioSourceStore(config.storage.audio_sources_file)
+audio_template_store = AudioTemplateStore(config.storage.audio_templates_file)
 value_source_store = ValueSourceStore(config.storage.value_sources_file)
 profile_store = ProfileStore(config.storage.profiles_file)
 
 # Migrate embedded audio config from CSS entities to audio sources
 audio_source_store.migrate_from_css(color_strip_store)
+# Assign default audio template to multichannel sources that have none
+audio_source_store.migrate_add_default_template(audio_template_store)
 
 processor_manager = ProcessorManager(
     picture_source_store=picture_source_store,
@@ -60,6 +65,7 @@ processor_manager = ProcessorManager(
     color_strip_store=color_strip_store,
     audio_source_store=audio_source_store,
     value_source_store=value_source_store,
+    audio_template_store=audio_template_store,
 )
 
 
@@ -104,6 +110,7 @@ async def lifespan(app: FastAPI):
         picture_target_store=picture_target_store,
         color_strip_store=color_strip_store,
         audio_source_store=audio_source_store,
+        audio_template_store=audio_template_store,
         value_source_store=value_source_store,
         profile_store=profile_store,
         profile_engine=profile_engine,