Add column headers to disk-usage output

Include the column headers (Original size, Compressed size, Deduplicated size)
in the disk-usage output for better readability.

Changes:
- Use grep -B1 to capture header line before "All archives"
- Display both header and data lines

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

scripts/disk-usage.sh

@@ -27,7 +27,7 @@ echo ""
 
 # Statistiques du repository Borg
 echo -e "${YELLOW}Espace utilisé par le repository Borg (toutes archives):${NC}"
-borgmatic info 2>/dev/null | grep "All archives" | grep GB | head -1
+borgmatic info 2>/dev/null | grep -B1 "All archives" | grep -E "Original size|All archives" | head -2
 
 echo ""