feat(processed-audio-sources): phase 6 - frontend source type cleanup

Rewrite audio source editor modal for capture/processed types only.
Remove old multichannel/mono/band_extract HTML sections and i18n keys.
Clean up legacy DOM section null-checks in audio-sources.ts.

plans/processed-audio-sources/CONTEXT.md

@@ -9,7 +9,7 @@
 - **Test:** `cd server && py -3.13 -m pytest tests/ --no-cov -q`
 
 ## Current State
-Phases 1-5 implemented. Phase 5 (Frontend Templates) added UI for managing audio processing templates.
+Phases 1-6 implemented. Phase 6 (Frontend Source Types) cleaned up the modal HTML and i18n keys for the new capture/processed source types.
 
 Phase 1 framework:
 - `AudioFilter` base class, `AudioFilterRegistry`, `AudioFilterOptionDef` in `core/audio/filters/`
@@ -95,7 +95,7 @@ _(none yet)_
 | Phase 3 | impl-agent | — | No | All 11 tasks done; channel/band logic deferred to Phase 4 |
 | Phase 4 | impl-agent | — | No | All 6 tasks done; dependency injection threaded through |
 | Phase 5 | impl-agent | — | No | 6/7 tasks done; Task 4 (preview) deferred to Phase 7 |
-| Phase 6 | — | — | — | — |
+| Phase 6 | impl-agent | — | No | Modal HTML + i18n cleanup; most tasks already done in Phases 3/5 |
 | Phase 7 | — | — | — | — |
 
 ## Environment & Runtime Notes