feat: Add cross-platform release pipeline with bundled Python backend

- PyInstaller packages web_server.py into standalone binary (no Python needed)
- Tauri sidecar: main.rs tries bundled binary first, falls back to system Python
- GitHub Actions workflow builds for macOS (arm64 + x64) and Windows (x64)
- Tauri shell plugin added for sidecar support
- externalBin injected only during CI build (dev mode uses system Python)
- Release creates draft with .dmg (macOS) and .msi/.exe (Windows)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>