feat: improve upgrade process with lock file and background execution

[SeyedHashtag]

This update ensures that the upgrade process completes reliably, even if the SSH connection drops, fully resolving the original issue.

Key Features:
-Resilience to SSH Disconnections: The upgrade runs under nohup in the background, allowing it to continue seamlessly after an SSH disconnect.
-Concurrent Execution Prevention: The lock file mechanism ensures that only one upgrade process can run at a time.
-Monitoring Support: Users can track progress using:
tail -f /var/log/hysteria_upgrade.log
-Status Verification: Users can check whether the upgrade is still active with:
ps aux | grep upgrade.sh
-Recovery From Interruptions: The system detects interrupted upgrades and handles them safely.
-Updated User Experience

When selecting option 3 — Update Panel, users now receive:
-A message indicating that the upgrade is running in the background
-The PID of the running upgrade process
-Instructions for monitoring and checking status
-Confirmation that they may safely disconnect their SSH session

⚠️ Note
These enhancements are not yet tested and require verification before production use.