ci(e2e): run WebKit, so the iOS guarantees actually gate a PR

The workflow installed only Chromium and ran chromium-desktop + chromium-mobile.
iOS Safari is the app's stated primary user — a wedding guest opening a QR link —
and WebKit is the only engine in the matrix that reproduces two of its behaviours:

  - it enforces X-Frame-Options on the hidden download iframe, so a site-wide DENY
    makes the keepsake download silently do nothing. Blink hands attachments to
    the download manager before the frame check and never notices.
  - it abandons a <video> load unless its Range probe gets a 206.

Both of those shipped. Adding 06-export to the webkit project in the round-1 fix
bought nothing on a PR, because CI never ran that project at all — the regression
test written specifically to catch the blocker only ever executed locally.

Runs 71 tests (67 pass, 4 skip on the documented IndexedDB-blob harness
limitation) in ~1.5 minutes locally, using the exact command added here.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/e2e.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   e2e:
-    name: Playwright E2E (chromium-desktop)
+    name: Playwright E2E (chromium + webkit)
     runs-on: ubuntu-latest
     timeout-minutes: 30
     steps:
@@ -25,7 +25,7 @@ jobs:
 
       - name: Install Playwright browsers
         working-directory: ./e2e
-        run: npx playwright install --with-deps chromium
+        run: npx playwright install --with-deps chromium webkit
 
       - name: Bring up the test stack
         working-directory: ./e2e
@@ -54,6 +54,22 @@ jobs:
         working-directory: ./e2e
         run: npm run test:e2e -- --project=chromium-mobile
 
+      # iOS Safari is the app's stated primary user (a wedding guest opening a QR link), and
+      # WebKit is the ONLY engine here that reproduces two of its behaviours:
+      #   - it enforces X-Frame-Options on the download iframe, so a site-wide `DENY` makes the
+      #     keepsake download silently do nothing. Blink hands attachments to the download
+      #     manager first and never notices. That shipped once already.
+      #   - it abandons a <video> load without a 206 response to its Range probe.
+      # Both regressions are invisible to every Chromium project, so running WebKit is what
+      # actually gates them on a PR rather than on someone remembering to test locally.
+      #
+      # The project is scoped in playwright.config.ts to the journeys a guest walks
+      # (01-auth, 02-upload, 03-feed, 06-export); four IndexedDB-blob tests skip themselves
+      # there — see helpers/webkit.ts for why that is the harness and not the app.
+      - name: Run E2E tests (webkit / iOS)
+        working-directory: ./e2e
+        run: npm run test:e2e -- --project=webkit-iphone
+
       - name: Upload Playwright report
         if: failure()
         uses: actions/upload-artifact@v4