feat: web UI protocol 2 client layer, JS and CSS bundles (step 2a)

- data-src/js: ProtoSchema (schema from the sync, short keys and names,
  defaults, cached by hash), ProtoModel (rooms, shades, groups, repeaters,
  header and status from the sync and the events) and ProtoClient (/ws,
  hello, sync with next, op() as a Promise of its reply, events, feeds,
  reconnect). Nothing uses them yet: 2b moves the UI onto them.
- minify.py joins js/*.js and index.js into index.js, and the three CSS
  files into app.css, so a page load opens fewer connections.
- test/js: node --test on the three modules, replaying the recorded
  fixtures; a CI job runs it and the firmware build waits for it.

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

.github/workflows/build.yaml

@@ -28,9 +28,26 @@ jobs:
       - name: Run native unit tests
         run: pio test -e native
 
+  webui:
+    name: Web UI tests
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v4
+
+      - name: Set up Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: "22"
+
+      # The protocol 2 client (data-src/js) against the recorded fixtures.
+      - name: Run web UI tests
+        run: node --test "test/js/*.test.js"
+
   build:
     # Don't build or publish firmware that fails its tests.
-    needs: test
+    needs: [test, webui]
     permissions: write-all
     name: ${{ matrix.name }}
     runs-on: ubuntu-latest