Add internationalization (i18n) support

[enoy19]

Goal

Ship multi-language UI while keeping type safety.

Library options

• typesafe-i18n – full TS inference, SvelteKit demo available ([github.com])
• sveltekit-i18n – tiny, no dependencies ([github.com])
• Paraglide 2.0 – new release, Svelte-friendly ([svelte.dev])

Acceptance criteria

• UI works in English and German.
• Missing keys fall back to English.
• TS strict mode passes.