Move mention popover to very top of screen for maximum visibility

- Changed from percentage-based to fixed pixel positioning
- Popover now appears at top: 50px from the very top of screen
- Should be clearly visible at the top of the window
- Horizontally centered for easy access

ui/desktop/src/components/EnhancedMentionPopover.tsx

@@ -249,7 +249,7 @@ const EnhancedMentionPopover = forwardRef<
       className="fixed z-50 bg-background-default border border-border-default rounded-lg shadow-lg min-w-80 max-w-md"
       style={{
         left: '50%', // Center horizontally
-        top: '20%', // Position in upper portion of screen
+        top: '50px', // Position near the very top of screen
         transform: 'translateX(-50%)', // Center the popover horizontally
       }}
     >