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/FirstPartyWidgets.qml

@@ -14,8 +14,7 @@ QtObject {
     "networkPanel":       { displayName: "Network",            description: "Wi-Fi list and connection state",            category: "Network",  allowMultiple: false, sourceDir: "../panels", sourceName: "Network" },
     "powerPanel":         { displayName: "Power",              description: "Battery, power profile, and system stats",    category: "System",   allowMultiple: false, sourceDir: "../panels", sourceName: "Power" },
     "bluetoothPanel":     { displayName: "Bluetooth",          description: "Bluetooth device list with connect/disconnect", category: "Network", allowMultiple: false, sourceDir: "../panels", sourceName: "Bluetooth" },
-    "clock":              { displayName: "Clock",              description: "Date / time text",                            category: "Time",     allowMultiple: false },
-    "daytime":            { displayName: "Daytime",            description: "Day/time label with date tooltip",            category: "Time",     allowMultiple: false, settingsForm: "daytimeSettings" },
+    "clock":              { displayName: "Clock",              description: "Day/time label; click to toggle alternate format", category: "Time",  allowMultiple: false, settingsForm: "clockSettings" },
     "indicators":         { displayName: "Indicators",         description: "Manual state indicators",                     category: "Status",   allowMultiple: false },
     "notificationCenter": { displayName: "Notification center", description: "Recent notifications + DND",                category: "Status",   allowMultiple: false },
     "systemUpdate":       { displayName: "System update",       description: "Indicates available system updates",         category: "System",   allowMultiple: false },