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/ViewModels/Modals/UsageMonitorModalViewModel.cs

@@ -40,6 +40,8 @@ public sealed partial class UsageMonitorModalViewModel : ViewModelBase
     [NotifyPropertyChangedFor(nameof(ModelsEmpty))]
     private bool _isBusy;
 
+    [ObservableProperty] private bool _isRefreshing;
+
     [ObservableProperty]
     [NotifyPropertyChangedFor(nameof(ModelsEmpty))]
     private IReadOnlyList<ModelUsageDisplayRow> _modelRows = Array.Empty<ModelUsageDisplayRow>();
@@ -92,6 +94,28 @@ public sealed partial class UsageMonitorModalViewModel : ViewModelBase
 
     private void OnUsageUpdated(UsageSnapshotDto snapshot) => Snapshot = snapshot;
 
+    /// <summary>
+    /// Manual refresh: the worker polls usage on a slow cadence (15 min idle / 5 min while a
+    /// task runs) to stay clear of the endpoint's 429s, so this is the way to get a number now.
+    /// </summary>
+    [RelayCommand]
+    private async Task RefreshAsync()
+    {
+        if (IsRefreshing) return;
+        IsRefreshing = true;
+        try
+        {
+            var snapshot = await _worker.RefreshUsageAsync();
+            if (snapshot is not null) Snapshot = snapshot;
+            await LoadUsageDataAsync();
+        }
+        catch (Exception ex)
+        {
+            ErrorReported?.Invoke(Loc.T("vm.usageMonitor.refreshFailed", ex.Message));
+        }
+        finally { IsRefreshing = false; }
+    }
+
     private void ApplyPresetRange(int days)
     {
         var today = DateOnly.FromDateTime(DateTime.Today);