Drop the daytime widget, use clock everywhere

Clock and daytime were near-duplicates with different click semantics
(clock toggles between formats on left-click and opens the timezone
selector on right-click; daytime was non-pressable with a hover tooltip).
Standardize on clock, since it's the more capable of the two.

- Delete widgets/daytime.qml and its FirstPartyWidgets entry.
- Migrate shell-defaults.json, shell.qml's builtin fallback, Bar.qml's
  fallback bar config, and the settings panel's builtin defaults so the
  center anchor and center-section entry both reference "clock".
- Rename the daytimeSettings form to clockSettings and rephrase
  "Tooltip format" → "Alternate format (click to toggle)" so the field
  label matches clock's actual behavior.
- Update the gallery dropdown options and bar/README docs.

shell/plugins/bar/widgets/daytime.qml

@@ -1,43 +0,0 @@
-import QtQuick
-import Quickshell
-import qs.Commons
-import qs.Ui
-
-BarWidget {
-  id: root
-  moduleName: "daytime"
-
-  property date now: new Date()
-
-  function formatLabel() {
-    if (!bar) return ""
-    var fmt = bar.vertical
-      ? String(setting("verticalFormat", "HH\n—\nmm"))
-      : String(setting("format", "dddd HH:mm"))
-    return Qt.formatDateTime(now, fmt)
-  }
-
-  function tooltipLabel() {
-    return Qt.formatDateTime(root.now, String(setting("formatAlt", "dd MMMM yyyy")))
-  }
-
-  implicitWidth: button.implicitWidth
-  implicitHeight: button.implicitHeight
-
-  SystemClock {
-    id: clockTimer
-    precision: SystemClock.Minutes
-    onDateChanged: root.now = clockTimer.date
-  }
-
-  WidgetButton {
-    id: button
-    anchors.fill: parent
-    bar: root.bar
-    text: root.formatLabel()
-    horizontalMargin: 8.75
-    verticalPadding: 8.75
-    tooltipText: root.tooltipLabel()
-    pressable: false
-  }
-}