When updating OpenChamber, stale openchamber-server processes from previous
versions can prevent the new version from starting. The app shows a loading
screen indefinitely with no error message.
This change adds a kill_stale_sidecar_processes() function that terminates
any existing openchamber-server processes before spawning a new one, ensuring
a clean startup every time.
- macOS/Linux: uses pkill -x for exact process name match
- Windows: uses taskkill /F /IM
Fixes startup issues after version updates.