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/Services/Interfaces/IWorkerClient.cs

@@ -139,4 +139,10 @@ public interface IWorkerClient : INotifyPropertyChanged
     Task SetOnlineInboxConfigAsync(OnlineInboxConfigInputDto input);
     Task SetOnlineInboxAuthAsync(string refreshToken);
     Task ClearOnlineInboxAuthAsync();
+
+    /// <summary>Raised whenever the worker's usage poller ticks (success or failure).</summary>
+    event Action<UsageSnapshotDto>? UsageUpdatedEvent;
+    Task<UsageSnapshotDto?> GetUsageSnapshotAsync();
+    Task<IReadOnlyList<ModelUsageRowDto>> GetModelUsageAsync(DateOnly from, DateOnly to);
+    Task<IReadOnlyList<TaskUsageRowDto>> GetTaskUsageAsync(DateOnly from, DateOnly to);
 }