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/MilestoneDetailPage.tsx

@@ -96,7 +96,7 @@ export default function MilestoneDetailPage() {
     currentMemberRole === 'admin'
   ))
 
-  const isProgressing = milestone?.status === 'progressing'
+  const isUndergoing = milestone?.status === 'undergoing'
 
   if (!milestone) return <div className="loading">Loading...</div>
 
@@ -117,7 +117,7 @@ export default function MilestoneDetailPage() {
       <div className="task-header">
         <h2>🏁 {milestone.title}</h2>
         <div className="task-meta">
-          <span className={`badge status-${milestone.status === 'progressing' ? 'in_progress' : milestone.status}`}>{milestone.status}</span>
+          <span className={`badge status-${milestone.status}`}>{milestone.status}</span>
           {milestone.due_date && <span className="text-dim">Due {dayjs(milestone.due_date).format('YYYY-MM-DD')}</span>}
           {milestone.planned_release_date && <span className="text-dim">Planned Release: {dayjs(milestone.planned_release_date).format('YYYY-MM-DD')}</span>}
         </div>
@@ -154,14 +154,14 @@ export default function MilestoneDetailPage() {
 
       <div className="section">
         <div style={{ display: 'flex', gap: 8, marginBottom: 16 }}>
-          {!isProgressing && canEditMilestone && (
+          {!isUndergoing && canEditMilestone && (
             <>
               <button className="btn-primary" onClick={() => { setActiveTab('tasks'); setShowCreateTask(true) }}>+ Create Task</button>
               <button className="btn-primary" onClick={() => { setActiveTab('supports'); setShowCreateSupport(true) }}>+ Create Support</button>
               <button className="btn-primary" onClick={() => { setActiveTab('meetings'); setShowCreateMeeting(true) }}>+ Schedule Meeting</button>
             </>
           )}
-          {isProgressing && <span className="text-dim">Milestone is in progress - cannot add new items</span>}
+          {isUndergoing && <span className="text-dim">Milestone is undergoing - cannot add new items</span>}
         </div>
 
         <MilestoneFormModal