[editor] Multi-language content support

- Database: `content_translations` table + `languages`/`default_language` columns on `websites`

- Editor: floating toolbar language switcher pill (only when 2+ languages enabled)

- Viewer: `?lang=de` URL param, `Accept-Language` detection, `hreflang` SEO tags

- Translatable content: text blocks, image captions/alt, page titles/descriptions, nav labels, site title

**Files:** Supabase migration, `src/hooks/use-translations.ts`, `EditorLayout.tsx`, `TextEditPanel.tsx`, `ImageEditPanel.tsx`