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.

tests/ClaudeDo.Worker.Tests/UiVm/TasksIslandViewModelPlanningTests.cs

@@ -107,6 +107,7 @@ sealed class FakeWorkerClient : IWorkerClient
     public event Action<string>? PlanningMergeAbortedEvent;
     public event Action<string>? PlanningCompletedEvent;
     public event Action<PrimeFiredEvent>? PrimeFired;
+    public event Action<UsageSnapshotDto>? UsageUpdatedEvent;
 #pragma warning restore CS0067
 
     public Task<MergeTargetsDto?> GetMergeTargetsAsync(string taskId) => Task.FromResult<MergeTargetsDto?>(null);
@@ -142,6 +143,12 @@ sealed class FakeWorkerClient : IWorkerClient
     public Task SetOnlineInboxAuthAsync(string refreshToken) => Task.CompletedTask;
     public Task ClearOnlineInboxAuthAsync() => Task.CompletedTask;
     public IReadOnlyList<ActiveTask> GetActiveTasks() => System.Array.Empty<ActiveTask>();
+
+    public Task<UsageSnapshotDto?> GetUsageSnapshotAsync() => Task.FromResult<UsageSnapshotDto?>(null);
+    public Task<IReadOnlyList<ModelUsageRowDto>> GetModelUsageAsync(DateOnly from, DateOnly to)
+        => Task.FromResult<IReadOnlyList<ModelUsageRowDto>>(System.Array.Empty<ModelUsageRowDto>());
+    public Task<IReadOnlyList<TaskUsageRowDto>> GetTaskUsageAsync(DateOnly from, DateOnly to)
+        => Task.FromResult<IReadOnlyList<TaskUsageRowDto>>(System.Array.Empty<TaskUsageRowDto>());
 }
 
 // ── Helper to build VM with pre-seeded Items ──────────────────────────────────