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/Bar/Widgets/Clock.qml

@@ -65,7 +65,7 @@ Rectangle {
           NText {
             visible: text !== ""
             text: modelData
-            family: useMonospacedFont ? Settings.data.ui.fontFixed : Settings.data.ui.fontDefault
+            family: Settings.data.ui.clockFont || (useMonospacedFont ? Settings.data.ui.fontFixed : Settings.data.ui.fontDefault)
             pointSize: {
               if (repeater.model.length == 1) {
                 return Style.fontSizeS * scaling
@@ -95,7 +95,7 @@ Rectangle {
           delegate: NText {
             visible: text !== ""
             text: modelData
-            family: useMonospacedFont ? Settings.data.ui.fontFixed : Settings.data.ui.fontDefault
+            family: Settings.data.ui.clockFont || (useMonospacedFont ? Settings.data.ui.fontFixed : Settings.data.ui.fontDefault)
             pointSize: Style.fontSizeS * scaling
             font.weight: Style.fontWeightBold
             color: usePrimaryColor ? Color.mPrimary : Color.mOnSurface