e2e: dump error-context.md to log on failure + bump upload-artifact

The Playwright run produces error-context.md per failed test
with a full DOM snapshot — useful for triaging UI test failures
without round-tripping through downloaded artifacts. Cat it
into the workflow log on failure.

Also bump actions/upload-artifact v3 → v4. v3 uploads still
return success on this Gitea runner but the artifacts don't
surface through the API or UI; v4 is the correct version per
the workflow header note.

.gitea/workflows/e2e.yml

@@ -96,6 +96,19 @@ jobs:
           docker cp e2e-pw:/work/playwright-report/. e2e/playwright/playwright-report/ || true
           docker cp e2e-pw:/work/test-results/. e2e/playwright/test-results/ || true
 
+      - name: Show Playwright failure context (on failure)
+        if: failure()
+        run: |
+          set +e
+          shopt -s nullglob globstar
+          for f in e2e/playwright/test-results/**/error-context.md; do
+            echo "::group::$f"
+            cat "$f"
+            echo "::endgroup::"
+          done
+          echo "Failure attachments (download via the playwright-report artifact):"
+          find e2e/playwright/test-results \( -name '*.png' -o -name '*.webm' -o -name 'trace.zip' \) -printf '  %p\n' | sort
+
       - name: Compose logs (on failure)
         if: failure()
         run: |
@@ -105,7 +118,7 @@ jobs:
 
       - name: Upload Playwright report (on failure)
         if: failure()
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: playwright-report
           path: |