ci(e2e): drop unused WebKit from Playwright install

The Playwright config only defines Chromium projects (chromium-desktop,
chromium-mobile), so WebKit was installed in CI but never launched by any
test. Installing it pulled WebKit's ~181-package / 114 MB gstreamer+ffmpeg
system-dependency stack from the Ubuntu archive on every run.

On run 27790385490 (main @ 028f15c) that apt download stalled on a slow
mirror and ran ~20 min until the job hit timeout-minutes: 20, surfacing as
the misleading "Error: The operation was canceled."

Install Chromium only. Removes the flaky/heavy install, speeds up every run,
and loses zero coverage since WebKit was never tested. Cache key updated to
drop the stale chromium-webkit binaries.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Paul Mulligan

.github/workflows/e2e.yml

@@ -39,15 +39,15 @@ jobs:
         id: playwright-cache
         with:
           path: ~/.cache/ms-playwright
-          key: playwright-chromium-webkit-${{ runner.os }}-${{ hashFiles('widget/pnpm-lock.yaml') }}
+          key: playwright-chromium-${{ runner.os }}-${{ hashFiles('widget/pnpm-lock.yaml') }}
 
       - name: Install Playwright browsers
         if: steps.playwright-cache.outputs.cache-hit != 'true'
-        run: pnpm exec playwright install --with-deps chromium webkit
+        run: pnpm exec playwright install --with-deps chromium
 
       - name: Install Playwright system deps
         if: steps.playwright-cache.outputs.cache-hit == 'true'
-        run: pnpm exec playwright install-deps chromium webkit
+        run: pnpm exec playwright install-deps chromium
 
       - name: Run E2E tests
         run: pnpm e2e