fix: defer setmon() after scene_surface creation to prevent segfault on window map

Root cause: applyrules() called setmon() which called resize(), but
resize() dereferences c->scene_surface->node before c->scene_surface
was created in mapnotify(). Fix by storing target monitor/tags in
pending_mon/pending_tags fields and calling setmon() after scene
nodes exist.

The crash affected all new windows, not just Firefox (which was
the first tested).

src/peachwm.c

@@ -492,7 +492,8 @@ static void applyrules(Client *c) {
   }
 
   c->isfloating |= client_is_float_type(c);
-  setmon(c, mon, newtags);
+  c->pending_mon = mon;
+  c->pending_tags = newtags;
 }
 
 static void client_arr_add(Client *c) {
@@ -956,10 +957,9 @@ static void commitnotify(struct wl_listener *listener, void *data) {
      * a wrong monitor.
      */
     applyrules(c);
-    if (c->mon) {
-      client_set_scale(client_surface(c), c->mon->wlr_output->scale);
+    if (c->pending_mon) {
+      client_set_scale(client_surface(c), c->pending_mon->wlr_output->scale);
     }
-    setmon(c, nullptr, 0); /* Make sure to reapply rules in mapnotify() */
 
     wlr_xdg_toplevel_set_wm_capabilities(
         c->surface.xdg->toplevel, WLR_XDG_TOPLEVEL_WM_CAPABILITIES_FULLSCREEN);
@@ -2289,6 +2289,9 @@ static void mapnotify(struct wl_listener *listener, void *data) {
     }
   }
 
+  /* Deferred setmon — scene nodes now exist */
+  setmon(c, c->pending_mon, c->pending_tags);
+
   if (c->isfullscreen)
     setfullscreen(c, 1);
   if (c->border_bg)