feat: add multi-language support for 20 locales

Add complete i18n infrastructure with 21 supported languages:
English, Simplified Chinese, Traditional Chinese, Japanese, Korean,
Spanish, French, Italian, Brazilian Portuguese, German, Dutch, Swedish,
Russian, Polish, Ukrainian, Arabic (RTL), Turkish, Hindi, Vietnamese,
Indonesian, and Thai.

- I18nProvider context with three-tier locale detection
  (user preference > navigator.languages > instance default > English)
- ~1500 translation keys per locale with TypeScript-enforced completeness
- Dynamic code-splitting: only the active locale is loaded at runtime
- Language selectors in footer, login page, settings, and mobile sidebar
- Arabic RTL support with CSS logical properties across all components
- Tool names, descriptions, and categories translated via i18n helpers
- Public API endpoint GET /api/v1/config/locale for instance default
- Multi-script font stack (CJK, Arabic, Devanagari, Thai, Cyrillic)
- format() and plural() helpers for interpolation and pluralization
- API error translation mapping (translateApiError)
- 36 Playwright e2e tests verifying all 21 locales load correctly
- 25 unit tests for format, plural, locale detection, and completeness
- Updated translations.md docs and CLAUDE.md conventions

apps/web/src/components/tools/pdf-to-image-preview.tsx

@@ -82,7 +82,7 @@ export function PdfToImagePreview() {
             {store.results.length} page
             {store.results.length !== 1 ? "s" : ""} converted
             {totalSize > 0 && (
-              <span className="text-muted-foreground font-normal ml-1">
+              <span className="text-muted-foreground font-normal ms-1">
                 ({formatSize(totalSize)})
               </span>
             )}
@@ -187,7 +187,7 @@ export function PdfToImagePreview() {
                 key={thumb.page}
                 type="button"
                 onClick={() => store.togglePage(thumb.page)}
-                className={`relative rounded-lg border overflow-hidden text-left transition-all ${
+                className={`relative rounded-lg border overflow-hidden text-start transition-all ${
                   isSelected
                     ? "border-primary ring-1 ring-primary/30"
                     : "border-border opacity-50 hover:opacity-75"