CI: the page under a headless browser on every push (tests/ui, the ui job)

A job of its own on ubuntu-24.04: Node 20, playwright-core 1.65.0 installed
into the runner's temp directory for the run (the repository keeps no
package.json), its own Chromium with the system libraries it needs, then
node tests/ui/run.mjs. Nothing is built and no server started.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -1,7 +1,8 @@
 # Checks on every push to main and dev and every pull request: formatting, lints, tests, the
-# page's scripts, the pins (tokens, CodeMirror), the public-repository hygiene -- and the app
-# started for real, on X11 and on Wayland, to prove it gets as far as its window. The packages
-# are built only by release.yml, from a tag.
+# page's scripts, the pins (tokens, CodeMirror, xterm), the public-repository hygiene, the page
+# itself under a headless browser (tests/ui) -- and the app started for real, on X11 and on
+# Wayland, to prove it gets as far as its window. The packages are built only by release.yml,
+# from a tag.
 name: ci
 on:
   push:
@@ -40,11 +41,32 @@ jobs:
         run: cargo test --locked
       - name: The page's scripts parse, and none sets HTML
         run: scripts/check-ui.sh
-      - name: The design tokens and CodeMirror are the server's, as pinned
+      - name: The design tokens, CodeMirror and xterm are the server's, as pinned
         run: |
           scripts/fetch-tokens.sh
           scripts/fetch-vendor.sh
 
+  # The page (ui/) under a headless Chromium, against a stand-in for the app's bridge
+  # (tests/ui/mock-tauri.js) with the server's answers as a table: nothing is built and no server
+  # is started. Playwright is the library only, installed for the run and not a dependency of
+  # the repository (it has no package.json), with the Chromium it was made for.
+  ui:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@v5
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+      - name: Playwright and its Chromium
+        run: |
+          npm install --no-save --no-package-lock --prefix "$RUNNER_TEMP/pw" playwright-core@1.65.0
+          node "$RUNNER_TEMP/pw/node_modules/playwright-core/cli.js" install --with-deps chromium
+      - name: The page under a headless browser
+        env:
+          NODE_PATH: ${{ runner.temp }}/pw/node_modules
+        run: node tests/ui/run.mjs
+
   # The app started for real: it must still be running after 25 seconds (timeout's 124), and its
   # log must not say it stopped. Once under Xvfb (X11) and once under a headless Weston (Wayland,
   # GTK's Wayland backend forced), each with a session bus (the tray and the notifications want