v3.40.0: Restore diagnostics and error reporting

Features:
- restore diagnose command for backup file analysis
- Deep COPY block verification for truncated dump detection
- PGDMP signature and gzip integrity validation
- Detailed error reports with --save-debug-log flag
- Ring buffer stderr capture (prevents OOM on 2M+ errors)
- Error classification with actionable recommendations

TUI Enhancements:
- Automatic dump validity safety check before restore
- Press 'd' in archive browser to diagnose backups
- Press 'd' in restore preview for debug log toggle
- Debug logs saved to /tmp on failure when enabled

Documentation:
- Updated README with diagnose command and examples
- Updated CHANGELOG with full feature list
- Updated restore preview screenshots

CONTRIBUTING.md

@@ -17,7 +17,7 @@ Be respectful, constructive, and professional in all interactions. We're buildin
 
 **Bug Report Template:**
 ```
-**Version:** dbbackup v3.2.0
+**Version:** dbbackup v3.40.0
 **OS:** Linux/macOS/BSD
 **Database:** PostgreSQL 14 / MySQL 8.0 / MariaDB 10.6
 **Command:** The exact command that failed