refactor(ui): redesign list settings and merge modals with custom chrome

Both modals now use SystemDecorations=None with a draggable title bar,
sectioned layout matching the rest of the island shell, Escape-to-cancel,
and themed brushes instead of hard-coded colours. ListSettings adds a
Browse... button that reads agent frontmatter from arbitrary .md files.

src/ClaudeDo.Ui/Views/Modals/ListSettingsModalView.axaml.cs

@@ -1,6 +1,8 @@
 using Avalonia.Controls;
+using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.Platform.Storage;
+using ClaudeDo.Data.Models;
 using ClaudeDo.Ui.ViewModels.Modals;
 
 namespace ClaudeDo.Ui.Views.Modals;
@@ -12,6 +14,63 @@ public partial class ListSettingsModalView : Window
         InitializeComponent();
     }
 
+    private void TitleBar_PointerPressed(object? sender, PointerPressedEventArgs e)
+    {
+        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
+            BeginMoveDrag(e);
+    }
+
+    private async void BrowseAgentClicked(object? sender, RoutedEventArgs e)
+    {
+        if (DataContext is not ListSettingsModalViewModel vm) return;
+        var top = TopLevel.GetTopLevel(this);
+        if (top is null) return;
+
+        var files = await top.StorageProvider.OpenFilePickerAsync(new FilePickerOpenOptions
+        {
+            Title = "Choose agent file",
+            AllowMultiple = false,
+            FileTypeFilter = new[]
+            {
+                new FilePickerFileType("Agent files (*.md)") { Patterns = new[] { "*.md" } },
+                new FilePickerFileType("All files") { Patterns = new[] { "*" } },
+            },
+        });
+        if (files.Count == 0) return;
+
+        var path = files[0].Path.LocalPath;
+        var existing = vm.Agents.FirstOrDefault(a => string.Equals(a.Path, path, StringComparison.OrdinalIgnoreCase));
+        if (existing is not null)
+        {
+            vm.SelectedAgent = existing;
+            return;
+        }
+
+        var (name, description) = ReadFrontmatter(path);
+        var agent = new AgentInfo(name, description, path);
+        vm.Agents.Add(agent);
+        vm.SelectedAgent = agent;
+    }
+
+    private static (string name, string description) ReadFrontmatter(string filePath)
+    {
+        var fallback = System.IO.Path.GetFileNameWithoutExtension(filePath);
+        try
+        {
+            using var reader = new System.IO.StreamReader(filePath);
+            if (reader.ReadLine()?.Trim() != "---") return (fallback, "");
+            string name = fallback, description = "";
+            while (reader.ReadLine() is { } line)
+            {
+                if (line.Trim() == "---") break;
+                if (line.StartsWith("name:")) name = line["name:".Length..].Trim();
+                else if (line.StartsWith("description:")) description = line["description:".Length..].Trim();
+            }
+            return (name, description);
+        }
+        catch { return (fallback, ""); }
+    }
+
     private async void BrowseClicked(object? sender, RoutedEventArgs e)
     {
         if (DataContext is not ListSettingsModalViewModel vm) return;