feat: update frontend type definitions and status enums to match new backend

- types/index.ts: Task status 'progressing' → 'undergoing' + 'completed'; Milestone status updated to open/freeze/undergoing/completed/closed + started_at field
- MilestoneFormModal: dropdown options updated
- MilestoneDetailPage: isProgressing → isUndergoing, badge class simplified
- MilestonesPage: badge class simplified
- TaskDetailPage: status transition map updated for new state machine

src/pages/MilestonesPage.tsx

@@ -51,7 +51,7 @@ export default function MilestonesPage() {
         {milestones.map((ms) => (
           <div key={ms.id} className="milestone-card" onClick={() => navigate(`/milestones/${ms.id}`)}>
             <div className="milestone-card-header">
-              <span className={`badge status-${ms.status === 'progressing' ? 'in_progress' : ms.status}`}>{ms.status}</span>
+              <span className={`badge status-${ms.status}`}>{ms.status}</span>
               <h3>{ms.title}</h3>
             </div>
             <p className="project-desc">{ms.description || 'No description'}</p>