chore(api,web): include validation message and detail in /api/generate error response

feat(web): play/stop button on each voice card; one plays at a time

fix: re-encode reference voices as PCM WAV in browser; expand turbo tag list to 9

feat(web): editorial-studio visual polish — Fraunces serif, IBM Plex mono, warm dark, ember accents

feat(web): Studio composes generate flow + SSE-driven loading banner

feat(web): UI components — ParamsPanel, TagBar, ModelPicker, DeviceBadge, VoiceLibrary, HistoryList, VoiceComposer

feat(web): API client, IndexedDB store, Recorder state machine

feat(web): Vite/React/TS/Tailwind/shadcn scaffold + Studio shell