Move conversation name to header (macOS document-title style)

The save indicator was sitting in the bottom-right corner of the footer.
Moved it to the center of the header bar, where macOS apps conventionally
show the document/conversation title. An orange dot appears when there are
unsaved changes; clicking saves. Removed the indicator from the footer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

oAI/Views/Main/FooterView.swift

@@ -93,15 +93,6 @@ struct FooterView: View {
             }
             #endif
 
-            // Save indicator (only when chat has messages)
-            if stats.messageCount > 0 {
-                SaveIndicator(
-                    conversationName: conversationName,
-                    hasUnsavedChanges: hasUnsavedChanges,
-                    onSave: onQuickSave
-                )
-            }
-
             // Update available badge (shows only when an update exists — no version number)
             #if os(macOS)
             UpdateBadge()