ci(desktop): build the renderer on PRs so vite breaks fail in CI

The desktop build break shipped because nothing in CI runs the
apps/desktop production build. typecheck only runs `tsc`, which does
not exercise Vite/Rolldown module resolution, so an unresolvable
package export (the @assistant-ui/tap "./react-shim" split) sailed
through green checks and only failed when users built from source on
install/update.

Add a desktop-build job that runs `npm run build` (tsc -b + vite build
+ assert-dist-built) for apps/desktop. This closes the gap so the same
class of break fails in CI instead of on every user's machine.

.github/workflows/typecheck.yml

@@ -23,3 +23,20 @@ jobs:
           cache: npm
       - run: npm ci
       - run: npm run --prefix ${{ matrix.package }} typecheck
+
+  # Production build of the desktop renderer. `typecheck` runs `tsc` only,
+  # which does NOT exercise Vite/Rolldown module resolution — so an
+  # unresolvable package export (e.g. a transitive @assistant-ui/tap that no
+  # longer exports "./react-shim") slips past typecheck and only explodes when
+  # users build apps/desktop from source on install/update. Run the real
+  # `vite build` here so that class of break fails in CI instead.
+  desktop-build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4
+        with:
+          node-version: 22
+          cache: npm
+      - run: npm ci
+      - run: npm run --prefix apps/desktop build