Prepare v2.2.1 question exchange and visual fixes

[LabSchool-GR]

What changed

• Adds secure UTF-8 CSV export/import for quiz questions, answer choices, and correct-answer positions.
• Enforces quiz ownership, teacher quotas, a 500-question safety cap, a 20-answer-column cap, UTF-8 validation, transactional imports, and spreadsheet-formula escaping.
• Reorganizes the question exchange interface into clear export and import workflows with localized Greek and English text.
• Prevents the unfiltered quiz background from flashing during question transitions in default, default_img, and modern_img.
• Updates the relevant bilingual documentation and screenshots.
• Prepares v2.2.1 release metadata and the incremental upgrade path from v2.2.0.
• Refreshes compatible frontend build dependencies to resolve the current npm advisories.

Why

Teachers need a practical way to transfer question banks between independently owned quizzes without sharing participant data or quiz access settings. The release also resolves the distracting background flash observed between questions and keeps the release packaging path aligned with the production v2.2.0 installation.

User and operator impact

• Quiz owners can export a reusable CSV and another teacher can import it into their own quiz.
• Images, participants, attempts, results, and access settings are intentionally excluded from CSV exchange.
• No database migration or Composer installation change is introduced in v2.2.1.
• The release workflow will generate both the full package and v2.2.0-to-v2.2.1 incremental package.

Validation

• php artisan test: 202 passed, 1129 assertions
• vendor/bin/pint --test: passed
• composer validate --strict: passed
• composer audit --locked: no advisories
• npm ci: passed
• npm audit --audit-level=high: 0 vulnerabilities
• npm run build: passed with Vite 6.4.3
• Blade cache compilation: passed
• git diff --check: passed