Expand headers area to show multiple rows

The headers scrolled window now fills available vertical space, allowing users to view and edit multiple headers simultaneously without scrolling.

src/window.py

@@ -545,6 +545,8 @@ class RosterWindow(Adw.ApplicationWindow):
 
         # Headers tab
         headers_scroll = Gtk.ScrolledWindow()
+        headers_scroll.set_vexpand(True)  # Expand to fill available space
+        headers_scroll.set_min_content_height(150)  # Show ~3-4 rows minimum
         self.headers_listbox = Gtk.ListBox()
         self.headers_listbox.add_css_class("boxed-list")
         headers_scroll.set_child(self.headers_listbox)