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/ViewModels/IslandsShellViewModel.cs

@@ -37,6 +37,7 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
     private readonly IDbContextFactory<ClaudeDoDbContext>? _dbFactory;
     private readonly Func<WorktreesOverviewModalViewModel> _worktreesOverviewVmFactory = () => null!;
     private readonly Func<WeeklyReportModalViewModel> _weeklyReportVmFactory = () => null!;
+    private readonly Func<UsageMonitorModalViewModel> _usageMonitorVmFactory = () => null!;
     private readonly Func<MergeModalViewModel> _mergeVmFactory = () => null!;
     private readonly Func<RepoImportModalViewModel>? _repoImportVmFactory;
 
@@ -207,6 +208,7 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
         IDbContextFactory<ClaudeDoDbContext> dbFactory,
         Func<WorktreesOverviewModalViewModel> worktreesOverviewVmFactory,
         Func<WeeklyReportModalViewModel> weeklyReportVmFactory,
+        Func<UsageMonitorModalViewModel> usageMonitorVmFactory,
         Func<MergeModalViewModel> mergeVmFactory,
         Func<RepoImportModalViewModel> repoImportVmFactory,
         MissionControlViewModel missionControl,
@@ -215,6 +217,7 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
         Lists = lists; Tasks = tasks; Details = details; Worker = worker;
         MissionControl = missionControl;
         UsagePill = usagePill;
+        UsagePill.OpenMonitorRequested += () => _ = OpenUsageMonitor();
         MissionControl.OpenInApp = id => _ = RevealTaskAsync(id);
         MissionControl.OpenSettingsRequested = () => Lists.OpenSettingsCommand.Execute(null);
         MissionControl.ErrorReported += FlashFooterError;
@@ -226,6 +229,7 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
         _dbFactory = dbFactory;
         _worktreesOverviewVmFactory = worktreesOverviewVmFactory;
         _weeklyReportVmFactory = weeklyReportVmFactory;
+        _usageMonitorVmFactory = usageMonitorVmFactory;
         _mergeVmFactory = mergeVmFactory;
         _repoImportVmFactory = repoImportVmFactory;
         Lists.SelectionChanged += (_, _) => Tasks.LoadForList(Lists.SelectedList);
@@ -429,6 +433,23 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
         finally { _weeklyReportOpen = false; }
     }
 
+    private bool _usageMonitorOpen;
+
+    [RelayCommand]
+    private async Task OpenUsageMonitor()
+    {
+        if (Dialogs is null || _usageMonitorOpen) return;
+        _usageMonitorOpen = true;
+        try
+        {
+            var vm = _usageMonitorVmFactory();
+            vm.ErrorReported += FlashFooterError;
+            await vm.LoadAsync();
+            await Dialogs.ShowUsageMonitorAsync(vm);
+        }
+        finally { _usageMonitorOpen = false; }
+    }
+
     [RelayCommand]
     private async Task CheckForUpdatesAsync()
     {