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.

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

@@ -145,6 +145,7 @@ sealed class FakeWorkerClient : IWorkerClient
     public IReadOnlyList<ActiveTask> GetActiveTasks() => System.Array.Empty<ActiveTask>();
 
     public Task<UsageSnapshotDto?> GetUsageSnapshotAsync() => Task.FromResult<UsageSnapshotDto?>(null);
+    public Task<UsageSnapshotDto?> RefreshUsageAsync() => 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)