Add port-in-use detection to start scripts with helpful error message

## Summary When starting the app with `./app/start.sh` or `./app/start.ps1`, if the target port is already in use (e.g. another worktree's backend is running), quart fails with a generic `OSError: [Errno 48] Address already in use`. The error doesn't tell the developer how to fix it. ## Motivation When working in multiple git worktrees of this repo simultaneously, the second instance collides on port 50505. After #3116 made the port configurable via `PORT` env var, the scripts should detect the conflict early and tell the developer exactly what to do. ## Proposed change Add a pre-flight port check in both start scripts before launching quart: **start.sh:** ```sh if lsof -i :"$port" -sTCP:LISTEN >/dev/null 2>&1; then echo "Port $port is already in use. Set PORT=<number> to use a different port." exit 1 fi ``` **start.ps1:** ```powershell if (Get-NetTCPConnection -LocalPort $port -State Listen -ErrorAction SilentlyContinue) { Write-Host "Port $port is already in use. Set env:PORT to use a different port." exit 1 } ``` ## Acceptance criteria - If the port is free, scripts behave exactly as today. - If the port is occupied, the script exits immediately with a clear message telling the user to set `PORT` to a different value.