feat(m6): board with threads/comments, markdown, tests, e2e

- BoardRepository (thread/comment CRUD, soft-delete, search, pagination,
  pinned-first ordering) + BoardService (membership permission,
  author/admin edit/delete, comment->board_commented notification,
  board_posted/comment_added activity logs, category validation)
- APIs: threads list/create/detail/edit/delete, comments create/edit/delete
- MarkdownBody (react-markdown + remark-gfm + rehype-sanitize),
  ThreadForm, CommentForm, board list + thread detail screens, ProjectNav link
- Unit tests (BoardRepository, BoardService) + e2e board.spec

components/board/MarkdownBody.tsx

@@ -0,0 +1,20 @@
+import ReactMarkdown from 'react-markdown';
+import remarkGfm from 'remark-gfm';
+import rehypeSanitize from 'rehype-sanitize';
+
+/**
+ * Markdown本文を安全にレンダリングする。
+ * HTML直接入力は無効化し、rehype-sanitize でサニタイズする。
+ */
+export function MarkdownBody({ bodyMd }: { bodyMd: string }) {
+  return (
+    <div className="prose prose-sm max-w-none">
+      <ReactMarkdown
+        remarkPlugins={[remarkGfm]}
+        rehypePlugins={[rehypeSanitize]}
+      >
+        {bodyMd}
+      </ReactMarkdown>
+    </div>
+  );
+}