fix(ui): render Plus and agent-settings gear icons

Icon.Plus was stroke-only geometry, invisible when used in a PathIcon
(fills). Author it as a filled cross, fixing every PathIcon use at once
(New-session, add-list, add-task). Replace the header bar's raw gear
TextBlock with the shared Icon.Settings PathIcon for consistency.

src/ClaudeDo.Ui/Views/Islands/Detail/TaskHeaderBar.axaml

@@ -65,7 +65,7 @@
             IsEnabled="{Binding AgentSettings.IsEnabled}"
             VerticalAlignment="Top"
             Margin="6,0,0,0">
-      <TextBlock Text="⚙" FontSize="{StaticResource FontSizeTaskTitle}"/>
+      <PathIcon Data="{StaticResource Icon.Settings}" Width="15" Height="15"/>
       <Button.Flyout>
         <Flyout Placement="BottomEdgeAlignedRight" ShowMode="Standard">
           <StackPanel Width="340" Spacing="10" Margin="4">