(SP: 3) [Frontend] Admin quiz upload + publish workflow

[LesiaUKR]

Goal

Enable admins to create new quizzes via JSON file upload and manage their lifecycle (draft -> ready -> active) through the admin panel UI.

Scope

• Migration 0015: add status column (draft/ready) to quizzes table
• Create quiz page at /admin/quiz/new with metadata form + JSON upload
• Category dropdown with inline creation (slug + title in 3 locales)
• Client-side JSON parsing and Zod validation
• Publish workflow with confirmation modals: Mark as Ready / Revert to Draft / Activate / Deactivate
• Server-side publish validation (all 3 locales for questions, answers, explanations)
• Metadata editor: title, description (3 locales), time limit
• Difficulty editing per question (beginner/medium/advanced)
• Upload additional question parts to draft quizzes
• Delete draft quizzes (only if inactive + zero attempts)
• Delete individual questions from draft quizzes
• Admin sidebar: New Quiz link

Expected impact

• New quizzes can be created and published without seed scripts or direct DB access
• Full content lifecycle management from admin UI
• Translation completeness enforced before publishing

Out of scope

• Form-based question creation
• Statistics dashboard
• Bulk operations (multi-select delete/publish)