tui: handle empty cells in cursor scan

When scanning the buffer for cursor position, also check empty cells.
The cursor might be positioned at an empty cell (e.g., end of line
or after all visible characters).

poc-agent/src/tui.rs

@@ -838,12 +838,17 @@ impl App {
                     // Count visible characters (skip zero-width and empty)
                     if !symbol.is_empty() {
                         if char_count == cursor_char_pos {
-                            // Found the cursor position
+                            // Found the cursor position - this is where the next char would go
                             cursor_x = x;
                             cursor_y = y;
                             break;
                         }
                         char_count += 1;
+                    } else if char_count == cursor_char_pos {
+                        // Empty cell but we've reached the cursor position
+                        cursor_x = x;
+                        cursor_y = y;
+                        break;
                     }
                 }
             }