fix(export-viewer): make the self-contained guard, and its spec, actually load-bearing

The font-inlining guard only caught a RENAME. It asked "did /fonts/<listed
family>.woff2 disappear?", so an ADDITION walked straight past it — and an
addition is the likelier accident: someone doing ordinary app work adds a
display font or a decorative background to the shared theme, has no reason to
open a viewer build config, and ships a keepsake that reaches for
/fonts/Playfair.woff2 on the guest's own disk. font-display: swap hides it, so
the artifact looks right to everyone who happens to have the file locally and
renders in Times New Roman for the couple.

It now asserts the invariant instead of a list: nothing in the emitted
keepsake may reference an external URL. Self-maintaining, and it covers fonts,
images and stylesheets alike. In writeBundle rather than generateBundle —
generateBundle runs more than once and the stylesheet is not inlined on the
earlier pass, so asserting there fails a perfectly good build.

viewer-no-broken-tiles gets a positive anchor. Its "nothing is broken" check
filters img elements, so a viewer that rendered NOTHING yields [] and passes:
the one spec whose whole subject is that the images resolve was the one that
would have stayed green through a total viewer regression. Everything else it
checks comes from the backend and the classic head script, neither of which
needs the viewer bundle to have run.

And a CI job, because neither of the above fires on its own: no workflow,
Dockerfile or script built this viewer, so the guard could sit disarmed
indefinitely, and the committed artifact — compiled into the binary with
include_dir! — could drift from its source with nothing to say so.

.github/workflows/checks.yml

@@ -102,6 +102,44 @@ jobs:
         working-directory: ./frontend
         run: npm run format:check
 
+  export-viewer:
+    name: Keepsake viewer — builds, self-contained, committed artifact in sync
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+          cache: 'npm'
+          cache-dependency-path: 'frontend/export-viewer/package-lock.json'
+
+      - name: Install deps
+        working-directory: ./frontend/export-viewer
+        run: npm ci || npm install
+
+      # Two things nothing else in CI covered, both of which ship a broken keepsake silently.
+      #
+      # 1. The build's own self-contained guard (`inlineThemeFonts`) is the only thing standing
+      #    between an added theme asset and a viewer that reaches for files on the guest's disk.
+      #    It is a build-time `this.error`, so it only fires when somebody runs this build — and
+      #    no workflow, Dockerfile or script did. It could sit disarmed indefinitely.
+      #
+      # 2. `backend/static/export-viewer/index.html` is COMMITTED and compiled into the binary with
+      #    `include_dir!`. A viewer source change merged without a manual rebuild ships the stale
+      #    artifact, and nothing anywhere would say so. `git diff --exit-code` is the check.
+      - name: Build the standalone viewer
+        working-directory: ./frontend/export-viewer
+        run: npm run build
+
+      - name: Committed artifact matches a clean rebuild
+        run: |
+          if ! git diff --exit-code -- backend/static/export-viewer/; then
+            echo "::error::backend/static/export-viewer/ is out of date with frontend/export-viewer/."
+            echo "Run 'npm run build' in frontend/export-viewer and commit the result."
+            exit 1
+          fi
+
   e2e-typecheck:
     name: E2E — typecheck + lint
     runs-on: ubuntu-latest