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/e2e/gui-navigation.spec.ts

@@ -892,13 +892,13 @@ test.describe("Routing Edge Cases", () => {
     await page.goto("/privacy");
 
     await expect(page.getByRole("heading", { name: "Privacy Policy" })).toBeVisible();
-    await expect(page.getByText("Back to app")).toBeVisible();
+    await expect(page.getByText("Back")).toBeVisible();
   });
 
-  test("/privacy Back to app link navigates home", async ({ loggedInPage: page }) => {
+  test("/privacy Back link navigates home", async ({ loggedInPage: page }) => {
     await page.goto("/privacy");
 
-    await page.getByText("Back to app").click();
+    await page.getByText("Back").click();
     await expect(page).toHaveURL("/");
   });