Consolidate OMARCHY_MENU_FONT lookups onto Style.font.menuFamily

Five plugins (menu, launcher, clipboard, emojis, polkit) each looked up
OMARCHY_MENU_FONT with the same env-or-monospace fallback. Hoist the
resolution into Style so plugins read one source of truth, and so the
fallback follows the real fontconfig family Style already tracks instead
of the literal string "monospace".

shell/plugins/menu/Menu.qml

@@ -32,7 +32,7 @@ Item {
     root.cancel()
   }
 
-  property string fontFamily: Quickshell.env("OMARCHY_MENU_FONT") || "monospace"
+  property string fontFamily: Style.font.menuFamily
   // JSONC menu definitions. The shell parses both at startup and merges
   // the user file on top of the defaults, so the keybind → IPC → visible
   // path doesn't have to shell out to bash + jq on every open.