test: add integration tests for installer warning detection

- add make target test-integration and run it in reusable CI workflow
- add integration unittest covering _page_warnings stderr output + deduplication
- surface Matomo installer warnings during Playwright flow (stderr only)

https://chatgpt.com/share/694c1371-365c-800f-bdf8-ede2e850e648

.github/workflows/reusable-test.yml

@@ -52,6 +52,21 @@ jobs:
           ruff check .
           ruff format --check .
 
+  integration:
+    runs-on: ubuntu-latest
+    needs: lint
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ inputs.python-version }}
+
+      - name: Integration tests
+        run: make test-integration
+
   e2e:
     runs-on: ubuntu-latest
     timeout-minutes: 30