✨ Feature: Add URL Validation and Pre-flight Checks

[dannycab]

Feature Request

Validate URLS.txt and configuration before starting long-running conversions.

Current Behavior

• Invalid URLs cause failures mid-batch
• No pre-flight validation
• Discover errors 30 minutes into conversion

Proposed Feature

wikiaccess validate --batch URLS.txt
✓ All 50 URLs are valid DokuWiki pages
✓ Wiki server is accessible
✓ Authentication working
⚠ 3 URLs return 404 (will be skipped)
✗ 2 URLs are not DokuWiki pages

Validation Checks

• URL format validation
• HTTP connectivity check
• DokuWiki detection (check for doku.php)
• Authentication verification
• Page existence check (HEAD request)
• Output directory writability
• Pandoc installation check
• pa11y installation check (if accessibility enabled)
• Disk space check

Tasks

• Create validation module
• Implement URL format validation
• Add connectivity checks
• Add DokuWiki detection
• Add authentication verification
• Create validate CLI command
• Auto-run validation before convert (with --skip-validation flag)