Fix: support macOS arm64 packaged webapp startup

[Acfufu]

Summary

• locate the ALAS root for packaged Electron launches instead of relying on process.cwd()
• keep the configured Deploy.Python.PythonExecutable when it exists
• add a macOS arm64 fallback that looks for a local alas conda environment when the default Windows ./toolkit/python.exe path is missing

Problem

Fixes #5105.

On macOS arm64, the packaged Electron app can start from Finder or another non-repo working directory. In that case the main process should not assume process.cwd() is the project root. The default Python path in config/deploy.yaml also points to ./toolkit/python.exe, which does not exist on Apple Silicon setups and causes spawn .../toolkit/python.exe ENOENT.

Issue #1873 already shows that Apple Silicon users need to provide their own Python environment. This change keeps the current config-first behavior, but lets the packaged app recover by using a local alas conda environment when the configured Windows path is unavailable.

Verification

• cd webapp && PATH=/Users/acfufu/.nvm/versions/node/v16.20.2/bin:$PATH npm run typecheck
• cd webapp && PATH=/Users/acfufu/.nvm/versions/node/v16.20.2/bin:$PATH npm run build
• cd webapp && PATH=/Users/acfufu/.nvm/versions/node/v16.20.2/bin:$PATH npm run compile
• launched webapp/dist/mac-arm64/alas.app/Contents/MacOS/alas from /tmp
• confirmed the packaged app responded on http://127.0.0.1:22267/ without toolkit/python.exe spawn errors

Notes

• the repository pre-push hook currently runs npm run typecheck from the repo root, but the only package.json is under webapp/, so that hook fails before checking the actual webapp workspace
• npm test still times out in the existing Electron smoke test while waiting for the first window, even though the packaged app and WebUI start successfully