ci(maestro): build flake packages so a stale pnpmDeps hash fails loudly

A web/pnpm-lock.yaml change invalidates the pinned pnpmDeps.hash in
flake.nix silently; nix build .#maestro-web then stays broken until
someone notices by hand (#98). Add a flake job that installs nix and
builds the packaged outputs on every CI run.

.gitea/workflows/ci.yml

@@ -97,3 +97,24 @@ jobs:
 
       - name: build
         run: pnpm -C web build
+
+  # The flake pins pnpmDeps.hash against web/pnpm-lock.yaml (#98). A lockfile
+  # change invalidates the pin silently elsewhere, so the packaged build has to
+  # fail here too.
+  flake:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install nix
+        run: |
+          curl -fsSL https://nixos.org/nix/install | sh -s -- --daemon --yes
+          echo "/nix/var/nix/profiles/default/bin" >> "$GITHUB_PATH"
+
+      - name: build flake packages
+        run: |
+          nix build \
+            .#webFrontend \
+            .#maestro \
+            .#maestro-web \
+            --no-link