Windows support for the packaged desktop app backend launcher

[regnull]

Goal

Make the packaged desktop app work on Windows, so make github-release can publish a Windows installer.

Context

electron-builder already has a Windows (NSIS) target, and pnpm electron:package --win builds. The installed app can't start its backend, though:

• frontend/electron/main.cjs starts the backend with spawn("/bin/bash", [script], …)
• scripts/electron-backend.sh is bash, and its uv lookup only knows Unix install locations

A likely approach is to move the launcher logic (finding uv, the environment defaults, UV_PROJECT_ENVIRONMENT, choosing root directory or app data) into Node inside the main process, which then works on every platform. Once Windows runs, add --win --x64 to scripts/github-release.sh.

Acceptance criteria

• The packaged app starts its backend on Windows 10/11 with uv installed (%USERPROFILE%\.local\bin\uv.exe and PATH)
• macOS and Linux behavior is unchanged, including OPENBOT_ROOT_DIRECTORY, DATABASE_URL and WORKSPACE_ROOT
• Unit tests for the launcher's environment and uv lookup
• The release script builds and uploads a Windows artifact; README updated

[Flobo2689x]

I'd like to take this. I'm on Windows 11, so I can test the actual packaged app, not just the logic. Plan:

• Move the launcher into frontend/electron/backend-launcher.cjs, as pure functions:
  • uv lookup: PATH incl. PATHEXT; %USERPROFILE%\.local\bin\uv.exe, ~/.cargo/bin, WinGet Links, Scoop shims on Windows; the current Unix locations elsewhere.
  • Environment defaults: DATABASE_URL, WORKSPACE_ROOT, TOOLS_DIR, LOG_FILE, UV_PROJECT_ENVIRONMENT, OPENBOT_ROOT_DIRECTORY vs app data, same precedence as electron-backend.sh.
  • The uv run … openbot.cli argv.
• main.cjs spawns uv directly. Stop uses the process group on macOS/Linux as today, and taskkill /T on Windows (no process groups there).
• Unit tests for the lookup and env. Then electron-backend.sh goes away, unless you'd rather keep it as a thin wrapper.
• Build a Windows package here and verify that the installed/unpacked OpenBot.exe starts its backend via uv. Add --win --x64 and the .exe asset to github-release.sh, and update the README.

I'll open a draft PR early so you can steer.