Use Color.foreground / Style.font.family for fallbacks

Five sites hardcoded "#cacccc" or "JetBrainsMono Nerd Font" as the
fallback when no bar context was available. Those magic strings are
the *current* palette defaults — they diverge silently when the theme
changes. Bind to the Color / Style singletons so the fallback tracks
whatever the theme says the foreground / monospace family should be.

Color.qml itself keeps the literal "#cacccc" — that's the seed value
the singleton is bootstrapped with before the theme loads.

shell/plugins/bar/widgets/systemStats.qml

@@ -233,8 +233,8 @@ BarWidget {
     property string title: ""
     property string value: ""
     property var history: []
-    property color barFg: "#cacccc"
-    property string fontFamily: "JetBrainsMono Nerd Font"
+    property color barFg: Color.foreground
+    property string fontFamily: Style.font.family
 
     spacing: Style.space(4)