fix: address all PR review critical and important issues

CRITICAL FIXES:
- Replace silent rollback failures with explicit error handling
- Add database-specific transaction syntax (BEGIN vs START TRANSACTION)
- Fix SQLFileParser chunk boundary handling for multi-char sequences
- Add line counting for manually advanced characters
- Add error logging for parse failures (no more silent failures)
- Add new ImportError types: rollbackFailed, foreignKeyCleanupFailed

IMPORTANT FIXES:
- Fix temp file leaks in ImportDialog preview
- Add temp file cleanup on dialog dismiss
- Add cancellation check after statement counting phase

DOCUMENTATION:
- Improve SQLFileParser header with algorithm explanation
- Document manual index advancement for multi-char sequences
- Add SQL escape convention comments (backslash vs doubled quotes)

All review issues addressed except code duplication (deferred as technical debt)

Author: datlechin