feat: switch frontend indexing to code-first identifiers

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/${ms.milestone_code || ms.id}`)}>
+          <div key={ms.id} className="milestone-item" onClick={() => ms.milestone_code && navigate(`/milestones/${ms.milestone_code}`)}>
             <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>}
@@ -136,7 +136,7 @@ export default function ProjectDetailPage() {
       <MilestoneFormModal
         isOpen={showMilestoneModal}
         onClose={() => setShowMilestoneModal(false)}
-        initialProjectId={project.id}
+        initialProjectCode={project.project_code || ''}
         lockProject
         onSaved={() => fetchProject()}
       />