Veyra Studio v0.4.0

feat: cross-platform installers and signed auto-updates

- GitHub Actions release workflow builds Windows (.exe/.msi), macOS
  (Apple silicon + Intel .dmg) and Linux (.AppImage/.deb/.rpm) on tag
  push, publishing a draft release with a signed latest.json
- In-app updater: checks on launch and every 6 hours, Help > Check for
  Updates, progress banner, saves work before restart
- Website: download page lists installers per OS from the latest
  release; /api/updates endpoint serves updates from our own server
- npm run release:version keeps package.json, tauri.conf.json and
  Cargo.toml in sync; docs/RELEASING.md explains the process
- Version 0.4.0

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>