feat: Implement forum feed layout and refactor space navigation logic to support new routing structure

src/app/features/forum/ForumFeedPage.tsx

@@ -0,0 +1,6 @@
+import { ForumBoardDetail } from './ForumBoardDetail';
+
+/** Forum post feed in the main pane (sidebar holds filters + post list). */
+export function ForumFeedPage() {
+  return <ForumBoardDetail />;
+}