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/lockKeys.qml

@@ -87,8 +87,8 @@ BarWidget {
     property bool active: false
 
     text: glyph
-    color: active ? (root.bar ? root.bar.foreground : "#cacccc") : Qt.rgba(0.7, 0.7, 0.7, 0.3)
-    font.family: root.bar ? root.bar.fontFamily : "JetBrainsMono Nerd Font"
+    color: active ? (root.bar ? root.bar.foreground : Color.foreground) : Qt.rgba(0.7, 0.7, 0.7, 0.3)
+    font.family: root.bar ? root.bar.fontFamily : Style.font.family
     font.pixelSize: Style.font.bodySmall
   }
 }