fix: prevent unmanaging maximized/fullscreen windows on UnmapNotify

- Modify handle_unmap_notify in src/main.c to ignore UnmapNotify events
  for windows with CLIENT_MAXIMIZED or CLIENT_FULLSCREEN flags.
- This prevents browsers and other applications from becoming 'lost'
  (unmanaged but mapped) when they temporarily unmap themselves during
  state transitions or resizing.

src/main.c

@@ -244,6 +244,10 @@ static void handle_unmap_notify(XUnmapEvent *ev)
         return;
     }
 
+    if (c->flags & (CLIENT_MAXIMIZED | CLIENT_FULLSCREEN)) {
+        return;
+    }
+
     client_unmanage(c);
 }