Add CI check for broken mattermost.com links in webapp (#35093)

* Add CI check for broken mattermost.com links in webapp

Add a script that scans the webapp source files for links to mattermost.com
domains and tests each unique URL for 404s. This helps detect broken
documentation and marketing links early.

- New script: webapp/scripts/check-external-links.mjs
- New npm target: check-external-links
- New CI job in webapp-ci.yml to run on every commit

* Add --markdown flag for GitHub Actions job summary

* Fix job summary: use pipefail and suppress progress output

* Require mattermost.com links to use /pl/ permalink format

* Require all mattermost.com links (including subdomains) to use /pl/

* Allow exceptions for push servers and root domain

* Make non-permalink URLs warnings instead of errors

* Add User-Agent header and retry GET on 403

* Follow redirects when checking URLs

Check the final destination of redirects to catch broken links that
redirect to error pages. If a redirect response has the Cloudflare
cf-mitigated header, assume the URL is OK and stop following.

* Simplify link checker code

- Combine PUSH_SERVER_PATTERN and HPNS_PATTERN into single regex
- Simplify validatePermalink to return boolean (reason was unused)
- Consolidate Cloudflare header checks in processResponse

* replace broken links with valid ones

* updates

.github/workflows/webapp-ci.yml

@@ -45,6 +45,23 @@ jobs:
         run: |
           npm run i18n-extract:check
 
+  check-external-links:
+    needs: check-lint
+    runs-on: ubuntu-24.04
+    timeout-minutes: 15
+    defaults:
+      run:
+        working-directory: webapp
+    steps:
+      - name: ci/checkout-repo
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      - name: ci/setup
+        uses: ./.github/actions/webapp-setup
+      - name: ci/check-external-links
+        run: |
+          set -o pipefail
+          npm run check-external-links -- --markdown | tee -a $GITHUB_STEP_SUMMARY
+
   check-types:
     needs: check-lint
     runs-on: ubuntu-24.04