fix: use CGMainDisplayID() instead of NSScreen.main for primary display detection (#21)

NSScreen.main returns the screen with keyboard focus (key window), not
the primary display with the menu bar. This caused items without an
explicit display to migrate to the wrong screen on display change
events. Use CGMainDisplayID() which always returns the menu bar display.

Author: typester

Cargo.lock

@@ -50,7 +50,7 @@ class RanmaAppDelegate: NSObject, NSApplicationDelegate {
             DisplayInfo(
                 id: screen.displayID,
                 name: screen.localizedName,
-                isMain: screen == NSScreen.main
+                isMain: screen.displayID == CGMainDisplayID()
             )
         }
         setDisplays(displays: displays)