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.Ui.Tests/ViewModels/UsageMonitorModalViewModelTests.cs

@@ -29,8 +29,19 @@ public class UsageMonitorModalViewModelTests
         public int ModelUsageCalls;
         public int TaskUsageCalls;
 
+        public UsageSnapshotDto? RefreshedSnapshot;
+        public int RefreshCalls;
+        public Exception? RefreshThrows;
+
         public override Task<UsageSnapshotDto?> GetUsageSnapshotAsync() => Task.FromResult(Snapshot);
 
+        public override Task<UsageSnapshotDto?> RefreshUsageAsync()
+        {
+            RefreshCalls++;
+            if (RefreshThrows is not null) throw RefreshThrows;
+            return Task.FromResult(RefreshedSnapshot);
+        }
+
         public override Task<IReadOnlyList<ModelUsageRowDto>> GetModelUsageAsync(DateOnly from, DateOnly to)
         {
             ModelUsageCalls++;
@@ -68,6 +79,60 @@ public class UsageMonitorModalViewModelTests
             isGateBlocked, gateReason, fetchedAtUtc ?? DateTime.UtcNow, isStale, lastError,
             configuredSlots, effectiveSlots, throttleBucket);
 
+    // ── Manual refresh ──────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task Refresh_ReplacesSnapshotAndReloadsTables()
+    {
+        var worker = new FakeWorker
+        {
+            Snapshot = Snapshot(new[] { Limit("session") }),
+            RefreshedSnapshot = Snapshot(new[] { Limit("session"), Limit("weekly_all") }),
+        };
+        var vm = new UsageMonitorModalViewModel(worker);
+        await vm.LoadAsync();
+        var callsAfterLoad = worker.ModelUsageCalls;
+
+        await vm.RefreshCommand.ExecuteAsync(null);
+
+        Assert.Equal(1, worker.RefreshCalls);
+        Assert.Equal(2, vm.GaugeRows.Count);
+        Assert.Equal(callsAfterLoad + 1, worker.ModelUsageCalls);
+        Assert.False(vm.IsRefreshing);
+    }
+
+    [Fact]
+    public async Task Refresh_NullResult_KeepsPreviousSnapshot()
+    {
+        var worker = new FakeWorker { Snapshot = Snapshot(new[] { Limit("session") }) };
+        var vm = new UsageMonitorModalViewModel(worker);
+        await vm.LoadAsync();
+
+        await vm.RefreshCommand.ExecuteAsync(null);
+
+        Assert.Single(vm.GaugeRows);
+    }
+
+    [Fact]
+    public async Task Refresh_Failure_ReportsErrorAndClearsBusyFlag()
+    {
+        var worker = new FakeWorker
+        {
+            Snapshot = Snapshot(new[] { Limit("session") }),
+            RefreshThrows = new InvalidOperationException("worker offline"),
+        };
+        var vm = new UsageMonitorModalViewModel(worker);
+        await vm.LoadAsync();
+        string? reported = null;
+        vm.ErrorReported += m => reported = m;
+
+        await vm.RefreshCommand.ExecuteAsync(null);
+
+        Assert.NotNull(reported);
+        Assert.Contains("worker offline", reported);
+        Assert.False(vm.IsRefreshing);
+    }
+
     // ── Gauge label derivation ──────────────────────────────────────────────
 
     [Fact]