Filter audio devices by engine type and update tutorial steps

- Add enumerate_devices_by_engine() returning per-engine device lists
  without cross-engine dedup so frontend can filter correctly
- API /audio-devices now includes by_engine dict alongside flat list
- Frontend caches per-engine data, filters device dropdown by selected
  template's engine_type, refreshes on template change
- Reorder getting-started tutorial: add API docs and accent color steps
- Fix tutorial trigger button focus outline persisting on step 2
- Use accent color variable for tutorial pulse ring animation

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

server/src/wled_controller/static/locales/en.json

@@ -222,8 +222,10 @@
   "tour.sources": "Sources — manage capture templates, picture sources, audio sources, and color strips.",
   "tour.profiles": "Profiles — group targets and automate switching with time, audio, or value conditions.",
   "tour.settings": "Settings — backup and restore configuration, manage auto-backups.",
+  "tour.api": "API Docs — interactive REST API documentation powered by Swagger.",
   "tour.search": "Search — quickly find and navigate to any entity with Ctrl+K.",
   "tour.theme": "Theme — switch between dark and light mode.",
+  "tour.accent": "Accent color — customize the UI accent color to your preference.",
   "tour.language": "Language — choose your preferred interface language.",
   "tour.restart": "Restart tutorial",
   "tour.dash.perf": "Performance — real-time FPS charts, latency metrics, and poll interval control.",