fix: disable NewMilestones button when no project selected

src/pages/MilestonesPage.tsx

@@ -56,7 +56,7 @@ export default function MilestonesPage() {
     <div className="milestones-page">
       <div className="page-header">
         <h2>🏁 Milestones ({milestones.length})</h2>
-        <button className="btn-primary" onClick={() => setShowCreate(!showCreate)}>
+        <button className="btn-primary" disabled={!projectFilter} onClick={() => setShowCreate(!showCreate)}>
           {showCreate ? 'Cancel' : '+ NewMilestones'}
         </button>
       </div>