Add test ID to group header for E2E test reliability

Update group header component to include data-testid for improved test targeting
and update Page Object Model to use the more reliable test ID selector.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/app/groups/[groupId]/group-header.tsx

@@ -16,7 +16,9 @@ export const GroupHeader = () => {
           {isLoading ? (
             <Skeleton className="mt-1.5 mb-1.5 h-5 w-32" />
           ) : (
-            <div className="flex">{group.name}</div>
+            <div className="flex" data-testid="group-name">
+              {group.name}
+            </div>
           )}
         </Link>
       </h1>