revert: remove i18n lazy loading and highlight.js selective import (#736)

Revert the dynamic import() for i18n locales and highlight.js core+
registration from #696. Dynamic imports create separate chunk files
that cause 404 errors for users after updating when the browser still
references old chunk hashes.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

tests/client/highlight-helper.test.ts

@@ -8,7 +8,7 @@ const highlightJsMock = vi.hoisted(() => ({
   registerLanguage: vi.fn(),
 }))
 
-vi.mock('highlight.js/lib/core', () => ({
+vi.mock('highlight.js', () => ({
   default: highlightJsMock,
 }))