fix: use valid HTML in summary element

Replace <h2> with <strong> inside <summary> for valid HTML structure.
Block-level elements are not allowed inside <summary>.

Author: david-waltermire

.github/workflows/build.yml

@@ -340,7 +340,7 @@ jobs:
       if: ${{ !inputs.skip_linkcheck && always() }}
       run: |
         echo "<details>" >> $GITHUB_STEP_SUMMARY
-        echo "<summary><h2>Link Checker Results</h2></summary>" >> $GITHUB_STEP_SUMMARY
+        echo "<summary><strong>Link Checker Results</strong></summary>" >> $GITHUB_STEP_SUMMARY
         echo "" >> $GITHUB_STEP_SUMMARY
         if [ -f "html-link-report.md" ]; then
           # Extract summary stats from the report (lychee markdown format uses "- [Status]")