Show merge-conflict badge on PR cards via GraphQL mergeable field

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DgBEW6qAwgn2fcQbA2f4ZQ

web/src/components/PrCard.tsx

@@ -38,6 +38,7 @@ export default function PrCard({
       </div>
       <div className="meta-row">
         {pr.isDraft && <span className="draft-tag">draft</span>}
+        {pr.mergeable === 'CONFLICTING' && <span className="conflict-tag">⚠ conflicts</span>}
         <span>{pr.author}</span>
         <span className="branch" title={pr.branch}>
           {pr.branch}