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/Ui/PanelSeparator.qml

@@ -1,4 +1,5 @@
 import QtQuick
+import qs.Commons
 
 // 1px horizontal divider for panel sections. The alpha-on-foreground tint
 // keeps the rule legible against the panel background without competing
@@ -6,7 +7,7 @@ import QtQuick
 Rectangle {
   id: root
 
-  property color foreground: "#cacccc"
+  property color foreground: Color.foreground
   property real strength: 0.12
 
   width: parent ? parent.width : implicitWidth