Cap the expanded comments pill at 600pt on wide panels

The excerpt stretches to the panel width, which on the iPad corner
layout's below-video panel meant a pill spanning the whole window. The
own-row expanded pill is now capped at 600pt, centered above the player
pill; the bottom-row variant already had a 400pt cap.

Yattee/Views/Player/PortraitDetailsPanel.swift

@@ -336,6 +336,9 @@ struct PortraitDetailsPanel: View {
                             showAvatar: panelSettings.commentsShowAvatars,
                             onTap: expandComments
                         )
+                        // Wide hosts (iPad corner/stacked panels) would
+                        // stretch the excerpt across the whole panel.
+                        .frame(maxWidth: 600)
                     }
 
                     // Bottom row