Add visual bar customizer

Each layout entry is now an object — {id, ...inline settings} — instead
of a (name, separate modules map) pair. Multiple instances of the same
widget are trivially supported (spacer is the main beneficiary). Settings
travel with the entry, so reordering keeps them attached.

shell.qml:
- normalizeLayout converts string entries to {id} and drops invalid ones
- ModuleSlot binds to a full entry; moduleName and moduleSettings derive
  from it via entryId/entrySettings helpers
- centerAnchor lookup walks layoutEntries directly
- builtinBarConfig defaults use the object form
- README + bar-defaults.json updated to the new schema

bar-settings/ (new Quickshell config):
- Top toolbar with Reset / Save and Position / centerAnchor dropdowns
- Three section editors (left/center/right) with current widgets shown
  as cards: name + description + move up/down/settings/remove buttons
- '+ Add widget' menu per section, sorted alphabetically by display name
  (only spacer can repeat; everything else is one per bar)
- Per-widget settings dialog (FloatingWindow) loads an inline schema for
  widgets in the catalog. Initial schemas: spacer (size), calendar
  (formats), brightness (step)
- FileView { atomicWrites } writes to ~/.config/omarchy/bar.json on Save

omarchy-launch-bar-settings + hypr window rules pin the settings window
to a 760px floating square. controlCenter gains a 'Customize bar…' button
that launches it.

default/quickshell/bar/widgets/controlCenter.qml

@@ -322,6 +322,16 @@ Item {
         color: Qt.rgba(root.bar.foreground.r, root.bar.foreground.g, root.bar.foreground.b, 0.12)
       }
 
+      Common.PillButton {
+        width: parent.width
+        iconText: "󰙪"
+        text: "Customize bar…"
+        foreground: root.bar.foreground
+        horizontalPadding: 10
+        verticalPadding: 8
+        onClicked: { root.run("omarchy-launch-bar-settings"); root.popupOpen = false }
+      }
+
       Row {
         width: parent.width
         spacing: 6