feat: add client-side dynamic i18n support to usage dashboard

[htqxd]

Summary

Integrates client-side dynamic internationalization (i18n) for the single-page dashboard (DASHBOARD_HTML in usage_dashboard.py), supporting Chinese (default), Vietnamese, and English.

Key enhancements:

• Client-side i18n: The translation dictionaries and localization logic are implemented via client-side JavaScript, maintaining the self-contained, offline capability of the single-file
  application.
• Auto-Refresh Protection: Wrapped all dynamically injected UI strings in the load() function with the translation helper t(). This prevents the 30-second periodic database refresh loop from
  reverting translated tables back to Chinese.
• Canvas & Number Formatting: Updates custom 2D canvas labels (including the 'unknown' model fallback) to match the selected language dynamically. Standardizes numeric formatting (e.g., thousands
  separators) using Intl.NumberFormat according to the active locale.
• FOUC (Flash of Untranslated Content) Prevention: Hides the page layout dynamically (opacity: 0) at initialization and restores visibility only after i18n processing is complete, preventing
  visual flashes of the default language.
• Robustness & Safety: Persists the user's language preference in localStorage. Wrapped storage access in a try-catch block and implemented language validation (ALLOWED_LANGS) to prevent
  frontend crashes in restrictive environments (e.g., private browsing, iframes).

Test plan

1. Compile the Python script to ensure zero syntax errors:

   python3 -m py_compile usage_dashboard.py

2. Run the dashboard server locally:
   python3 usage_dashboard.py serve
3. Open http://127.0.0.1:8320/ in a browser:
   - Verify that the page auto-detects your browser language on the initial load.
   - Change the language using the toolbar selector to Vietnamese, English, and Chinese. Confirm that static headers, KPIs, and table columns translate immediately.
   - Check the horizontal model consumption chart to confirm that the fallback label translates correctly (no English 'unknown' leak).
   - Wait 30 seconds or click "Refresh" to verify that newly loaded table rows do not revert to Chinese.
   - Reload the page (F5) to verify that your selected language persists via localStorage.