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/Commons/Style.qml

@@ -268,9 +268,19 @@ QtObject {
     return (isFinite(n) && n > 0) ? Math.round(n) : fallback
   }
 
+  // The launcher, menu, polkit, emojis, and clipboard surfaces honor an
+  // OMARCHY_MENU_FONT override for users who want a different family on the
+  // summoned popups than on the bar. Resolved once at startup; an empty env
+  // value falls back to the shared fontconfig alias.
+  readonly property string menuFontFamily: {
+    var override = Quickshell.env("OMARCHY_MENU_FONT")
+    return (override && override.length > 0) ? override : fontFamily
+  }
+
   readonly property QtObject font: QtObject {
     readonly property string family: root.fontFamily
     readonly property string resolvedFamily: root.resolvedFontFamily
+    readonly property string menuFamily: root.menuFontFamily
     readonly property int baseSize: root.fontBaseSize
 
     readonly property int caption:      root.fontToken("caption",       root.fontPx(0.833))   // 10