Use toolkit downloads folder instead of /tmp or /root

Better approach per user suggestion:
- Downloads to: /root/server-toolkit/downloads/acronis-install-YYYYMMDD-HHMMSS/
- Keeps toolkit directory organized
- Avoids polluting /root
- Avoids /tmp noexec issues
- Added downloads/ to .gitignore
- Cleanup removes timestamped installation directory after completion

Benefits:
- All downloads in one place
- Easy to find if debugging needed
- Cleaner than scattered in /root
- Still allows execution (not in /tmp)

.gitignore

@@ -54,3 +54,4 @@ id_ed25519.pub
 # Config files that might contain sensitive data
 config.local.*
 *.credentials
+downloads/