feat: Add Audiobook tab with chunked generation, preview, and Story auto-save

[omercelik]

Summary

This PR adds an end-to-end Audiobook workflow in the app for long-form TXT narration, including quick preview, sentence-aware chunking, chunk retry controls, and automatic Story creation/updating during generation.

Changes

• Added new Audiobook UI and workflow at voicebox/app/src/components/AudiobookTab/AudiobookTab.tsx
• Added sentence-aware text chunking utility at voicebox/app/src/lib/utils/textChunking.ts
• Wired new route and sidebar tab:
  • voicebox/app/src/router.tsx
  • voicebox/app/src/components/Sidebar.tsx
• Added visible scrollbar utility styles:
  • voicebox/app/src/index.css
• Extended generation request typing with optional instruct:
  • voicebox/app/src/lib/api/types.ts
• Updated release notes:
  • voicebox/CHANGELOG.md
• Included updated generated/asset files:
  • voicebox/bun.lock
  • voicebox/tauri/src-tauri/gen/Assets.car
  • voicebox/tauri/src-tauri/gen/voicebox.icns

Testing

Automated checks run

• bunx @biomejs/biome check app/src/components/Sidebar.tsx app/src/index.css app/src/lib/api/types.ts app/src/router.tsx app/src/components/AudiobookTab/AudiobookTab.tsx app/src/lib/utils/textChunking.ts CHANGELOG.md
• cd voicebox/app && bun run build

Known unrelated baseline issue

• bun run build:web fails in existing code outside this PR scope (pre-existing TypeScript errors).

PR Checklist (from CONTRIBUTING.md)

• Code follows style guidelines
• Documentation updated (changelog)
• Changes tested
• No breaking changes introduced in this patch
• CHANGELOG.md updated