fix(usage): stop 429s with an activity-dependent poll cadence + manual refresh

The usage monitor polled the undocumented OAuth usage endpoint every 60s and
earned 429s. It now polls every 5 min while any task is Running and every
15 min while idle (usage_poll_interval_active_seconds / _idle_seconds, both
clamped to >= 60; the old single usage_poll_interval_seconds key is gone).

A 429 comes back as UsageRateLimitedException carrying Retry-After and adds
exponential backoff on top, capped at 30 min and never shorter than the normal
cadence; the strike count resets on the first success. The schedule arithmetic
is the pure static UsagePollSchedule.NextDelay.

Since the idle cadence is slow on purpose, WorkerHub.RefreshUsage drives
UsageMonitorService.RefreshNowAsync behind a Refresh now button in the Usage
Monitor modal: an out-of-band poll that pushes the loop's next-due time out so
no double poll follows, with a 10s cooldown so click-spam can't earn a 429.

Staleness now measures against the slower (idle) interval so an idle worker
isn't flagged stale just for not polling.

src/ClaudeDo.Ui/Services/Interfaces/IWorkerClient.cs

@@ -143,6 +143,9 @@ public interface IWorkerClient : INotifyPropertyChanged
     /// <summary>Raised whenever the worker's usage poller ticks (success or failure).</summary>
     event Action<UsageSnapshotDto>? UsageUpdatedEvent;
     Task<UsageSnapshotDto?> GetUsageSnapshotAsync();
+
+    /// <summary>Forces an out-of-band usage poll on the worker and returns the fresh snapshot.</summary>
+    Task<UsageSnapshotDto?> RefreshUsageAsync();
     Task<IReadOnlyList<ModelUsageRowDto>> GetModelUsageAsync(DateOnly from, DateOnly to);
     Task<IReadOnlyList<TaskUsageRowDto>> GetTaskUsageAsync(DateOnly from, DateOnly to);
 }