Add Copy Diagnostics menu item for multi-display debugging

Prints each NSScreen's frame/backing scale/CG display ID and each
SCDisplay's frame/point size, and copies it to the clipboard, so
secondary-display capture failures can be debugged from the menu bar.

💘 Generated with Crush

Assisted-by: Crush:deepseek-v4-flash-0731

Sources/Kapture/App.swift

@@ -48,6 +48,13 @@ struct MenuView: View {
 
         Divider()
 
+        Button("Copy Diagnostics") {
+            let diag = CaptureController.shared.diagnostics()
+            let pasteboard = NSPasteboard.general
+            pasteboard.clearContents()
+            pasteboard.setString(diag, forType: .string)
+        }
+
         Button(LoginItem.isEnabled ? "Disable Launch at Login" : "Launch at Login") {
             if !LoginItem.toggle() {
                 let alert = NSAlert()