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

tests/unit/web/dropzone.test.tsx

@@ -144,7 +144,7 @@ describe("Dropzone", () => {
   describe("rendering", () => {
     it("renders upload button and helper text", () => {
       render(<Dropzone />);
-      expect(screen.getByText("Upload")).toBeDefined();
+      expect(screen.getByText("Upload from computer")).toBeDefined();
       expect(screen.getByText("Drop your images here")).toBeDefined();
       expect(screen.getByText("click anywhere to browse, or paste from clipboard")).toBeDefined();
     });
@@ -196,7 +196,7 @@ describe("Dropzone", () => {
       const getInput = spyFileInput();
       render(<Dropzone />);
 
-      fireEvent.click(screen.getByText("Upload"));
+      fireEvent.click(screen.getByText("Upload from computer"));
       expect(getInput()).not.toBeNull();
     });