fix: resolve CSS shorthand/non-shorthand property conflict

Replace border shorthand with borderWidth, borderStyle, borderColor
to avoid React warning when borderColor is overridden in focused state

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/UserSelect.tsx

@@ -166,7 +166,9 @@ const styles: Record<string, React.CSSProperties> = {
     alignItems: 'center',
     justifyContent: 'space-between',
     padding: '10px',
-    border: '1px solid #ddd',
+    borderWidth: '1px',
+    borderStyle: 'solid',
+    borderColor: '#ddd',
     borderRadius: '4px',
     backgroundColor: 'white',
     cursor: 'pointer',