feat(ui): add usage monitor modal with gauges and model/task usage analysis

Adds a Usage Monitor modal (Worker menu + wired to the footer/Mission-Control
usage pill's Open command): dynamic gauges built from UsageSnapshotDto.Limits
with gate-threshold marks, a stale/blocked-gate band, and Models/Tasks tabs
backed by GetModelUsageAsync/GetTaskUsageAsync over a 7d/30d/custom range.

src/ClaudeDo.Ui/Views/WindowDialogService.cs

@@ -58,6 +58,13 @@ public sealed class WindowDialogService : IDialogService
         await dlg.ShowDialog(_owner);
     }
 
+    public async Task ShowUsageMonitorAsync(UsageMonitorModalViewModel vm)
+    {
+        var dlg = new UsageMonitorModalView { DataContext = vm };
+        vm.CloseAction = () => dlg.Close();
+        await dlg.ShowDialog(_owner);
+    }
+
     public async Task ShowSettingsAsync(SettingsModalViewModel vm)
     {
         var dlg = new SettingsModalView { DataContext = vm };