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/ProjectDetailPage.tsx

@@ -92,7 +92,7 @@ export default function ProjectDetailPage() {
           <div className="member-list">
             {members.map((m) => (
               <span key={m.id} className="badge" style={{ marginRight: 8 }}>
-                {`User #${m.user_id} (${m.role})`}
+                {`${m.username || m.full_name || `User #${m.user_id}`} (${m.role})`}
                 {canEditProject && (
                   <button
                     onClick={(e) => { e.stopPropagation(); removeMember(m.user_id, m.role) }}