Consolidate omarchy-shell colors into a single shell.toml surface

Five plugins each owned their own colors.toml watcher with subtly
different parsers, plus two ad-hoc per-theme override files
(notifications.json, image-picker-colors.json). This collapses all of
that into one source of truth: Commons/Color.qml watches colors.toml +
shell.toml and exposes Color.bar.*, Color.popups.*, Color.notifications.*,
Color.menu.*, Color.imagePicker.* for every surface to bind to.

shell.toml is generated by the existing template pipeline from
default/themed/shell.toml.tpl. Themes can ship their own shell.toml to
override individual keys; everything missing falls back to the
foundational palette via root.pick(). Settings panel is intentionally
not themable beyond the foundational tokens.

last-horizon and solitude ship a minimal shell.toml to preserve their
historical 'notification border matches Hyprland active border'
behavior, which previously came from parsing the theme's hyprland.conf
(now removed).

default/quickshell/omarchy-shell/plugins/bar/common/PopupCard.qml

@@ -1,6 +1,7 @@
 import QtQuick
 import Quickshell
 import Quickshell.Hyprland
+import qs.Commons
 
 PopupWindow {
   id: root
@@ -89,8 +90,8 @@ PopupWindow {
   Rectangle {
     id: card
     anchors.fill: parent
-    color: root.bar ? root.bar.background : "#101315"
-    border.color: root.bar ? root.bar.foreground : "#cacccc"
+    color: Color.popups.background
+    border.color: Color.popups.border
     border.width: 1
     radius: 0
     opacity: root.open ? 1.0 : 0