ci: typecheck and build the GUI frontend

[ColeMei]

CI ran fmt/check/clippy/test but never touched the frontend — no npm ci, no npm run build. TypeScript errors and Vite/tauri.conf.json breakage were caught only by hand, which is a gap the manual GUI smoke-test checklist was quietly absorbing.

Adds a gui job running npm ci + npm run build (which is tsc && vite build, so it typechecks as well as bundles).

• Runs on ubuntu-latest: nothing in the frontend build is macOS-specific, and Linux runners start faster.
• Stops short of a full tauri build on purpose — the Rust side of the app is already covered by the rust job's workspace-wide check/clippy, and bundling costs minutes rather than seconds.
• npm cache keyed on crates/openwith-gui/package-lock.json.

Verified npm ci succeeds against the committed lockfile.