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/WorkerClient.cs

@@ -61,6 +61,8 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
     public event Action<string>? RefineStartedEvent;
     public event Action<string, bool, string?>? RefineFinishedEvent;
 
+    public event Action<UsageSnapshotDto>? UsageUpdatedEvent;
+
     public event Action<string, string>? PlanningMergeStartedEvent;
     public event Action<string, string>? PlanningSubtaskMergedEvent;
     public event Action<string, string, IReadOnlyList<string>>? PlanningMergeConflictEvent;
@@ -202,6 +204,9 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
             Dispatcher.UIThread.Post(() => RefineStartedEvent?.Invoke(id)));
         _hub.On<string, bool, string?>("RefineFinished", (id, ok, err) =>
             Dispatcher.UIThread.Post(() => RefineFinishedEvent?.Invoke(id, ok, err)));
+
+        _hub.On<UsageSnapshotDto>("UsageUpdated", snapshot =>
+            Dispatcher.UIThread.Post(() => UsageUpdatedEvent?.Invoke(snapshot)));
     }
 
     public Task StartAsync()
@@ -574,6 +579,15 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
     public async Task ClearOnlineInboxAuthAsync()
         => await _hub.InvokeAsync("ClearOnlineInboxAuth");
 
+    public Task<UsageSnapshotDto?> GetUsageSnapshotAsync()
+        => TryInvokeAsync<UsageSnapshotDto>("GetUsageSnapshot");
+
+    public async Task<IReadOnlyList<ModelUsageRowDto>> GetModelUsageAsync(DateOnly from, DateOnly to)
+        => await TryInvokeAsync<List<ModelUsageRowDto>>("GetModelUsage", from, to) ?? [];
+
+    public async Task<IReadOnlyList<TaskUsageRowDto>> GetTaskUsageAsync(DateOnly from, DateOnly to)
+        => await TryInvokeAsync<List<TaskUsageRowDto>>("GetTaskUsage", from, to) ?? [];
+
     // IWorkerClient explicit implementations (drop typed return values)
     async Task IWorkerClient.StartPlanningSessionAsync(string taskId, CancellationToken ct)
         => await StartPlanningSessionAsync(taskId, ct);
@@ -676,3 +690,46 @@ public sealed record OnlineInboxConfigInputDto(
     string ClientId,
     string Scopes,
     string RedirectUri);
+
+public sealed record UsageLimitDto(
+    string Kind,
+    string Group,
+    double Percent,
+    string Severity,
+    DateTimeOffset? ResetsAt,
+    string? ScopeModelDisplayName,
+    bool IsActive);
+
+public sealed record UsageSnapshotDto(
+    double? FiveHourPercent,
+    DateTimeOffset? FiveHourResetsAt,
+    double? SevenDayPercent,
+    DateTimeOffset? SevenDayResetsAt,
+    IReadOnlyList<UsageLimitDto> Limits,
+    int FiveHourThresholdPct,
+    int SevenDayThresholdPct,
+    bool IsGateBlocked,
+    string? GateReason,
+    DateTime? FetchedAtUtc,
+    bool IsStale,
+    string? LastError);
+
+public sealed record ModelUsageRowDto(
+    DateOnly Date,
+    string Model,
+    string Scope,
+    long InputTokens,
+    long OutputTokens,
+    long CacheReadTokens,
+    long CacheCreationTokens,
+    int Messages);
+
+public sealed record TaskUsageRowDto(
+    string TaskId,
+    string TaskTitle,
+    string ListId,
+    string ListName,
+    string? Model,
+    int Runs,
+    long TokensIn,
+    long TokensOut);