bundle UI files into the binary

This is optional but now enabled for release builds.

Why?

* It shrinks the release docker images a bit, as the binary
  includes only the gzipped version of files and uncompressed into RAM
  at startup (which should be fast).

* It's a step toward #160.

.github/workflows/ci.yml

@@ -36,6 +36,9 @@ jobs:
         # The retry here is to work around "Unable to connect to azure.archive.ubuntu.com" errors.
         # https://github.com/actions/runner-images/issues/6894
         run: sudo apt-get --option=APT::Acquire::Retries=3 update && sudo apt-get --option=APT::Acquire::Retries=3 install libncurses-dev libsqlite3-dev pkgconf
+      - uses: actions/setup-node@v2
+        with:
+          node-version: 18
       - name: Install Rust
         uses: actions-rs/toolchain@v1
         with:
@@ -43,8 +46,13 @@ jobs:
           toolchain: ${{ matrix.rust }}
           override: true
           components: ${{ matrix.extra_components }}
+      - run: cd ui && npm ci
+      - run: cd ui && npm run build
       - name: Test
-        run: cd server && cargo test ${{ matrix.extra_args }} --all
+        run: |
+          cd server
+          cargo test ${{ matrix.extra_args }} --all
+          cargo test --features=bundled-ui ${{ matrix.extra_args }} --all
         continue-on-error: ${{ matrix.rust == 'nightly' }}
       - name: Check formatting
         if: matrix.rust == 'stable'