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/Services/IDialogService.cs

@@ -16,6 +16,7 @@ public interface IDialogService
 {
     Task ShowAboutAsync(AboutModalViewModel vm);
     Task ShowWeeklyReportAsync(WeeklyReportModalViewModel vm);
+    Task ShowUsageMonitorAsync(UsageMonitorModalViewModel vm);
     Task ShowSettingsAsync(SettingsModalViewModel vm);
     Task ShowListSettingsAsync(ListSettingsModalViewModel vm);
     Task ShowRepoImportAsync(RepoImportModalViewModel vm);