feat(ui): PowerUse-Toggle im Usage-Monitor, Opt-in pro Liste

Der Toggle sitzt unter den Gauges im Usage-Monitor, weil "das restliche Fenster
ausnutzen" eine Entscheidung ist, die man beim Blick auf das restliche Fenster
trifft. Er speichert sofort per Read-modify-write, gleiche Disziplin wie
SaveStagesAsync, damit er keine Felder ueberschreibt, die er nicht besitzt.

Zwei Stellen, die das Opt-in still ausgeschaltet haetten:
- WorkerHub.UpdateListConfig loescht die Config-Zeile, wenn alle Felder leer
  sind. PowerUseAllowed muss in diese Leer-Pruefung, sonst verschwindet das
  Opt-in sofort wieder, wenn es das einzige gesetzte Feld ist.
- SettingsModalViewModel baut das AppSettingsDto positional neu auf. Ohne
  Round-trip haette jedes Speichern der Settings PowerUseEnabled auf den
  Record-Default false zurueckgesetzt. Gleiches Muster wie _throttleStages.

Das Handoff-Budget ist als Spinner in Settings -> General editierbar, weil es
auch fuer normale (interaktive) Handler-Runs gilt und nicht an PowerUse haengt.
Das Addendum selbst ist ueber Settings -> Files anpassbar.

src/ClaudeDo.Ui/ViewModels/Modals/UsageMonitorModalViewModel.cs

@@ -100,6 +100,45 @@ public sealed partial class UsageMonitorModalViewModel : ViewModelBase
     [ObservableProperty]
     private IReadOnlyList<UsageGaugeRowViewModel> _gaugeRows = Array.Empty<UsageGaugeRowViewModel>();
 
+    /// <summary>
+    /// PowerUse master switch. It lives here rather than in Settings because turning it on is a
+    /// "spend the rest of the window" decision, made while looking at how much window is left.
+    /// On its own it changes nothing — a list-handler run only goes unattended when its LIST also
+    /// opted in (list settings → PowerUse), so this can stay on without affecting other lists.
+    /// </summary>
+    [ObservableProperty] private bool _powerUseEnabled;
+
+    // Set while LoadAsync writes the stored value in, so seeding the property doesn't immediately
+    // write it straight back to the worker.
+    private bool _loadingPowerUse;
+
+    partial void OnPowerUseEnabledChanged(bool value)
+    {
+        if (_loadingPowerUse) return;
+        _ = SavePowerUseAsync(value);
+    }
+
+    /// Read-modify-write, same discipline as SaveStagesAsync: never clobber a field this modal
+    /// doesn't own.
+    private async Task SavePowerUseAsync(bool value)
+    {
+        try
+        {
+            var settings = await _worker.GetAppSettingsAsync();
+            if (settings is null)
+            {
+                ErrorReported?.Invoke(Loc.T("vm.usageMonitor.powerUseSaveFailed", Loc.T("vm.settingsModal.workerOffline")));
+                return;
+            }
+
+            await _worker.UpdateAppSettingsAsync(settings with { PowerUseEnabled = value });
+        }
+        catch (Exception ex)
+        {
+            ErrorReported?.Invoke(Loc.T("vm.usageMonitor.powerUseSaveFailed", ex.Message));
+        }
+    }
+
     public bool IsStale => Snapshot?.IsStale == true;
     public string? LastError => Snapshot?.LastError;
     public bool IsGateBlocked => Snapshot?.IsGateBlocked == true;
@@ -148,6 +187,15 @@ public sealed partial class UsageMonitorModalViewModel : ViewModelBase
             Snapshot = snapshot;
             _worker.UsageUpdatedEvent -= OnUsageUpdated;
             _worker.UsageUpdatedEvent += OnUsageUpdated;
+
+            var settings = await _worker.GetAppSettingsAsync();
+            if (settings is not null)
+            {
+                _loadingPowerUse = true;
+                try { PowerUseEnabled = settings.PowerUseEnabled; }
+                finally { _loadingPowerUse = false; }
+            }
+
             ApplyPresetRange(SelectedPresetDays);
             await LoadUsageDataAsync();
         }