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/Modals/UsageMonitorModalView.axaml.cs

@@ -0,0 +1,8 @@
+using Avalonia.Controls;
+
+namespace ClaudeDo.Ui.Views.Modals;
+
+public partial class UsageMonitorModalView : Window
+{
+    public UsageMonitorModalView() => InitializeComponent();
+}