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/CommentForm.tsx

@@ -0,0 +1,65 @@
+'use client';
+
+import { useState, type FormEvent } from 'react';
+import { useRouter } from 'next/navigation';
+
+export function CommentForm({
+  projectId,
+  threadId,
+}: {
+  projectId: number;
+  threadId: number;
+}) {
+  const router = useRouter();
+  const [bodyMd, setBodyMd] = useState('');
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  async function onSubmit(event: FormEvent<HTMLFormElement>) {
+    event.preventDefault();
+    setLoading(true);
+    setError(null);
+    const res = await fetch(
+      `/api/projects/${projectId}/board/threads/${threadId}/comments`,
+      {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ bodyMd }),
+      }
+    );
+    setLoading(false);
+    if (res.ok) {
+      setBodyMd('');
+      router.refresh();
+    } else {
+      const b = (await res.json().catch(() => null)) as {
+        error?: { message?: string };
+      } | null;
+      setError(b?.error?.message ?? '投稿に失敗しました');
+    }
+  }
+
+  return (
+    <form onSubmit={onSubmit} className="space-y-2">
+      <textarea
+        placeholder="コメント（Markdown）"
+        value={bodyMd}
+        onChange={(e) => setBodyMd(e.target.value)}
+        className="min-h-[80px] w-full rounded border px-3 py-2"
+        required
+      />
+      {error && (
+        <p className="text-sm text-red-600" role="alert">
+          {error}
+        </p>
+      )}
+      <button
+        type="submit"
+        disabled={loading}
+        className="rounded bg-blue-600 px-4 py-2 text-sm font-medium text-white hover:bg-blue-700 disabled:opacity-50"
+      >
+        {loading ? '投稿中...' : 'コメント投稿'}
+      </button>
+    </form>
+  );
+}