Fix macOS quit: intercept applicationShouldTerminate: to allow real exit

GLFW's applicationShouldTerminate: converts all quit paths (Cmd+Q, dock,
menu bar) into window close requests and returns NSTerminateCancel, so our
close-to-hide intercept was hiding the window instead of letting the app exit.

Fix by implementing applicationShouldTerminate: in ReopenDelegate, setting a
darwinQuitting flag before forwarding to GLFW's delegate. The close intercept
checks this flag and calls Window.Close() instead of Hide() when quitting.

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

ui/app_delegate_darwin.m

@@ -3,6 +3,7 @@
 #import <AppKit/AppKit.h>
 
 extern void appReopened(void);
+extern void appShouldTerminate(void);
 
 @interface ReopenDelegate : NSObject<NSApplicationDelegate>
 @property (nonatomic, strong) id<NSApplicationDelegate> wrapped;
@@ -11,6 +12,16 @@ extern void appReopened(void);
 
 @implementation ReopenDelegate
 
+- (NSApplicationTerminateReply)applicationShouldTerminate:(NSApplication *)sender {
+    // Set the quitting flag before GLFW's delegate fires close requests for
+    // each window, so our close intercept knows to close rather than hide.
+    appShouldTerminate();
+    if ([self.wrapped respondsToSelector:_cmd]) {
+        return [self.wrapped applicationShouldTerminate:sender];
+    }
+    return NSTerminateNow;
+}
+
 - (BOOL)applicationShouldHandleReopen:(NSApplication *)app
                     hasVisibleWindows:(BOOL)hasVisible {
     if (!hasVisible) {