Fix: milestone routes use milestone_code, config check via backend

- Milestone navigation now uses milestone_code instead of numeric id
- MilestoneDetailPage uses milestone.id (numeric) for project-scoped API calls
- App.tsx checks /config/status on backend first, falls back to wizard
- Added milestone_code to Milestone type
- Fixed MilestoneDetailPage to use fetched milestone.id for sub-queries

src/pages/ProjectDetailPage.tsx

@@ -114,7 +114,7 @@ export default function ProjectDetailPage() {
           {canEditProject && <button className="btn-sm" onClick={() => setShowMilestoneModal(true)}>+ New</button>}
         </h3>
         {milestones.map((ms) => (
-          <div key={ms.id} className="milestone-item" onClick={() => navigate(`/milestones/${ms.id}`)}>
+          <div key={ms.id} className="milestone-item" onClick={() => navigate(`/milestones/${ms.milestone_code || ms.id}`)}>
             <span className={`badge status-${ms.status === 'open' ? 'open' : ms.status === 'closed' ? 'closed' : 'in_progress'}`}>{ms.status}</span>
             <span className="milestone-title">{ms.title}</span>
             {ms.due_date && <span className="text-dim"> · Due {dayjs(ms.due_date).format('YYYY-MM-DD')}</span>}