feat(ui): add usage pill to footer and mission control header

Adds the IWorkerClient/WorkerClient usage surface (GetUsageSnapshot,
GetModelUsage, GetTaskUsage, UsageUpdated event) and a shared
UsagePillViewModel hosted once in IslandsShellViewModel (footer) and
once in MissionControlViewModel (header), showing "5h X% · 7d Y%"
with warn/blocked/stale states via existing design tokens. The
OpenMonitorCommand is wired but currently a no-op, pending the usage
monitor modal.

src/ClaudeDo.Ui/Views/MainWindow.axaml

@@ -2,6 +2,7 @@
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:vm="using:ClaudeDo.Ui.ViewModels"
         xmlns:islands="using:ClaudeDo.Ui.Views.Islands"
+        xmlns:controls="using:ClaudeDo.Ui.Views.Controls"
         xmlns:converters="using:ClaudeDo.Ui.Converters"
         xmlns:loc="using:ClaudeDo.Ui.Localization"
         x:Class="ClaudeDo.Ui.Views.MainWindow"
@@ -184,6 +185,12 @@
           </StackPanel>
         </Button>
 
+        <!-- Left: usage pill (click opens the usage monitor, wired in a later subtask) -->
+        <controls:UsagePill DockPanel.Dock="Left"
+                             DataContext="{Binding UsagePill}"
+                             Margin="14,0,0,0"
+                             VerticalAlignment="Center"/>
+
         <!-- Right: worker log line — click to open the Log Visualizer overlay -->
         <Button DockPanel.Dock="Right"
                 Command="{Binding OpenLogVisualizerCommand}"