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/polkit/PolkitAgent.qml

@@ -8,7 +8,7 @@ import qs.Commons
 Item {
   id: root
 
-  property string fontFamily: Quickshell.env("OMARCHY_MENU_FONT") || "monospace"
+  property string fontFamily: Style.font.menuFamily
   // Bound to the central [polkit] section in shell.toml via Color.qml.
   property color accent: Color.polkit.accent
   property color background: Color.polkit.background