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/Views/Modals/UsageMonitorModalView.axaml

@@ -60,6 +60,18 @@
         </Border>
       </StackPanel>
 
+      <!-- Refresh: the worker polls on a slow cadence to avoid the endpoint's 429s -->
+      <StackPanel DockPanel.Dock="Top" Orientation="Horizontal" Spacing="8"
+                  Margin="20,12,20,0" VerticalAlignment="Center">
+        <Button Classes="btn" Content="{loc:Tr modals.usageMonitor.refresh}"
+                Command="{Binding RefreshCommand}"
+                IsEnabled="{Binding !IsRefreshing}"/>
+        <Ellipse Classes="spinner" Width="14" Height="14" VerticalAlignment="Center"
+                 IsVisible="{Binding IsRefreshing}"/>
+        <TextBlock Classes="meta" VerticalAlignment="Center"
+                   Text="{loc:Tr modals.usageMonitor.refreshHint}"/>
+      </StackPanel>
+
       <!-- Gauges -->
       <ItemsControl DockPanel.Dock="Top" Margin="20,12,20,0" ItemsSource="{Binding GaugeRows}">
         <ItemsControl.ItemsPanel>