Quill Desktop v0.2.0

ci: add GitHub Actions workflow for desktop build & release

Automated multi-platform builds triggered on version tags (v*):

- macOS: aarch64 (Apple Silicon) + x64 (Intel) → .dmg
- Windows: x64 → .exe (NSIS installer)
- Linux: amd64 → .AppImage + .deb

Uses tauri-action for builds, creates draft release, then publishes
after all platforms succeed. Supports manual dispatch with tag input.

Requires secrets: TAURI_SIGNING_PRIVATE_KEY (optional, for signed builds)

Co-Authored-By: Claude Opus 4.6 <<EMAIL>>