feat: add i18n option with a dedicated route per language, including german translation

[sha-oai]

This PR adds the structure for i18n and german version of the website under /de path. Further language implementations will be under their specific path. The UI was not changed, except the token replacement for interface texts that need translations.

Summary

• Added basic i18n plumbing with route-aware language detection (src/app/[lang]/page.tsx, src/lib/i18n.ts, src/hooks/useI18n.ts) and persisted language choice in the Zustand store (src/lib/store.ts).
• Localized UI strings across header, buttons, dialogs, and TTS controls using the new t() helper (e.g., src/components/TTSPage.tsx, src/components/ShareDialog.tsx, src/components/ui/*).
• Refactored the voice/vibe library into a translatable base plus locale overlays (src/lib/library/base.ts, src/lib/library/index.ts) and shipped full German copy in src/lib/library/lang/de.ts; updated store logic to localize presets without overwriting user-edited text.
• Kept client rendering for the TTS page but now pass the initial language from dynamic routes to keep
  HTML lang in sync (src/components/ClientDynamicTTS.tsx).

Adding More Languages:

To add another locale,

1. extend Language/SUPPORTED_LANGUAGES in src/lib/i18n.ts and supply message strings
2. add a translation map for presets in src/lib/library/lang/<code>.ts and register it in TRANSLATIONS in src/lib/library/index.ts;
3. add the route param to generateStaticParams so /fr (etc.) prebuilds;
4. optionally seed locale-specific copy for any new UI strings and voices. This keeps routing, storage, and preset localization working automatically.