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/TaskDetailPage.tsx

@@ -63,9 +63,10 @@ export default function TaskDetailPage() {
   if (!task) return <div className="loading">Loading...</div>
 
   const statusActions: Record<string, string[]> = {
-    open: ['progressing', 'closed'],
-    pending: ['progressing', 'closed'],
-    progressing: ['pending', 'closed'],
+    open: ['undergoing', 'closed'],
+    pending: ['open', 'closed'],
+    undergoing: ['completed', 'closed'],
+    completed: ['open'],
     closed: ['open'],
   }