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

vitest.config.ts

@@ -85,6 +85,7 @@ export default defineConfig({
       "@": path.resolve(__dirname, "apps/web/src"),
       "framer-motion": path.join(landingNodeModules, "framer-motion"),
       "@snapotter/image-engine": path.resolve(__dirname, "packages/image-engine/src/index.ts"),
+      "@snapotter/shared/i18n": path.resolve(__dirname, "packages/shared/src/i18n"),
       "@snapotter/shared": path.resolve(__dirname, "packages/shared/src/index.ts"),
       fastify: path.join(apiNodeModules, "fastify"),
       "@fastify/cookie": path.join(apiNodeModules, "@fastify/cookie"),