feat: add i18n support for EN / TA / HI / FR to QyverixAI frontend

[SarthakKharche]

Summary

Adds full i18n (internationalisation) support to the QyverixAI frontend so all UI text can be displayed in the user's chosen language without a page reload. A TRANSLATIONS object in frontend/index.html holds strings for four locales — English (default), Tamil, Hindi, and French. A language selector dropdown in the nav lets users switch locale at any time; the choice is persisted in localStorage and restored on reload. All text-bearing elements are marked with data-i18n attributes and updated via a single applyTranslations() call on switch. Accessible labels on key controls are synced alongside visible text. No external libraries were introduced and no existing UI behaviour was changed.

Fixes #133

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor

Testing

• I ran backend tests (pytest -q)
• I tested frontend behavior manually

Switched to each of the four locales and verified all labelled strings updated.
Reloaded the page after switching — confirmed locale was restored from localStorage.
Cleared localStorage and confirmed English loaded as the default.
Verified toast messages rendered in the active locale.
Confirmed no existing interactions were broken by the change.

Checklist

• Code is readable and beginner-friendly
• No fake or placeholder behavior introduced
• Documentation updated if needed

Notes:
Default language is English.
The selected language is restored on reload.
Existing UI behavior is unchanged aside from localization.

[imDarshanGK]

@SarthakKharche Please link the related issue number in the PR description (e.g., Fixes #issue_number), resolve the conflicts, and include screenshots or implementation proof so the changes can be verified easily.

[SarthakKharche]

@imDarshanGK
I have successfully edited the PR description with Fix Number. Also I have resolved the conflicts and have attached the screenshots of the work.
Please review it once.

Thank You

[imDarshanGK]

@SarthakKharche Please split the unrelated PR-analysis workflow change into a separate PR. This localization PR should stay focused on [index.html]) only. Once that extra workflow change is removed or reviewed separately, I can re-check the i18n update

[SarthakKharche]

@imDarshanGK
I have removed the pr-analysis file from this branch. Plz review it.