v0.1.0

feat: rename crate to vkit, add release CI, theme/sync/preview polish

- Rename crate + binary from vkit-rs to vkit (crates.io name available)
- Add GitHub Actions: release.yml (cross-platform binaries + cargo publish) and ci.yml (fmt/clippy/test)
- Add vsix download/install, editor sync, and preview features
- Refine auto light/dark theme detection and shared widgets