feat: designer window is not draggable from an implicit title-bar strip

The main window has no title bar, so opt out of the CSD title-bar move
band (csd_titlebar_move -> false). Moving the window is now reserved for
explicitly-declared drag handles (a title bar or panel), of which there
are none yet. Resize borders are unaffected.

Co-Authored-By: Claude Fable 5 <[email protected]>

 src/application.rs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/application.rs b/src/application.rs
index 892df41..5bb5388 100644
--- a/src/application.rs
+++ b/src/application.rs
@@ -316,6 +316,13 @@ impl Application for State {
         !self.is_detached_network
     }
 
+    /// No title bar to grab, so the window is not implicitly draggable. Moving it is reserved
+    /// for explicitly-declared drag handles (a title bar or panel), of which there are none
+    /// yet — see `is_movable_backplate_at` (left at its `false` default).
+    fn csd_titlebar_move(&self) -> bool {
+        false
+    }
+
     fn cursor_icon(&self, x: f32, y: f32) -> Option<CursorIcon> {
         if !self.is_detached_network {
             return None;