feat: code-first migration — replace raw IDs with codes and usernames

- TaskDetailPage: show task_code instead of raw #id in header
- TaskDetailPage: show author_username in comment metadata
- ProjectDetailPage: show member username instead of User #id
- ProposeDetailPage: show created_by_username instead of User #id
- TasksPage: fix status filter options to match actual statuses
  (pending/open/undergoing/completed/closed)
- TasksPage: fix status color map for correct status values
- Types: add username/full_name to ProjectMember, author_username
  to Comment, created_by_username to Propose
- Supports TODO §3.1 (code-first UI migration)

src/pages/ProposeDetailPage.tsx

@@ -138,7 +138,7 @@ export default function ProposeDetailPage() {
         <div className="detail-grid">
           <div><strong>Propose Code:</strong> {propose.propose_code || '—'}</div>
           <div><strong>Status:</strong> {propose.status}</div>
-          <div><strong>Created By:</strong> User #{propose.created_by_id || '—'}</div>
+          <div><strong>Created By:</strong> {propose.created_by_username || (propose.created_by_id ? `User #${propose.created_by_id}` : '—')}</div>
           <div><strong>Created:</strong> {dayjs(propose.created_at).format('YYYY-MM-DD HH:mm')}</div>
           <div><strong>Updated:</strong> {propose.updated_at ? dayjs(propose.updated_at).format('YYYY-MM-DD HH:mm') : '—'}</div>
           <div><strong>Feature Task:</strong> {propose.feat_task_id || '—'}</div>