Add clock font setting for customizable clock displays

This commit adds a new 'Clock Font' setting that allows users to customize
the font used specifically for clock displays in the bar and widgets,
independent of the default UI font.

Features:
- New clockFont property in Settings.data.ui (defaults to 'Roboto')
- Updated Bar Clock widget to use the custom font with fallback support
- Added searchable font dropdown in General Settings tab
- Backward compatible - uses default font if clockFont is not set
- Real-time updates - changes apply immediately

The font selection uses FontService.availableFonts and includes proper
fallback logic that respects the existing monospaced font setting.

Modules/Settings/Tabs/GeneralTab.qml

@@ -237,6 +237,20 @@ ColumnLayout {
         }
       }
 
+      NSearchableComboBox {
+        label: "Clock Font"
+        description: "Font used specifically for clock displays in the bar and widgets."
+        model: FontService.availableFonts
+        currentKey: Settings.data.ui.clockFont
+        placeholder: "Select clock font..."
+        searchPlaceholder: "Search fonts..."
+        popupHeight: 420 * scaling
+        minimumWidth: 300 * scaling
+        onSelected: function (key) {
+          Settings.data.ui.clockFont = key
+        }
+      }
+
       ColumnLayout {
         NLabel {
           label: I18n.tr("settings.general.fonts.default.scale.label")