Add board view/edit mode separation with cancel button.

/boards/[id] is now view mode (read-only) with an edit button.
/boards/[id]/edit is edit mode with save and cancel buttons.
Save navigates back to view, cancel discards changes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/(main)/boards/[boardId]/edit/page.tsx

@@ -0,0 +1,12 @@
+import type { Metadata } from 'next';
+import { BoardPage } from '../BoardPage';
+
+export default async function ({ params }: { params: Promise<{ boardId: string }> }) {
+  const { boardId } = await params;
+
+  return <BoardPage boardId={boardId} editing />;
+}
+
+export const metadata: Metadata = {
+  title: 'Edit Board',
+};