feat(ui): add subtask tree view with expand/collapse in task list

Tasks with subtasks show a chevron for inline expand/collapse.
Subtask checkboxes toggle completion state directly. Also sets
Windows AppUserModelID for proper taskbar identity.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ClaudeDo.App/Program.cs

@@ -8,14 +8,21 @@ using ClaudeDo.Ui.ViewModels;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.DependencyInjection;
 using System;
+using System.Runtime.InteropServices;
 
 namespace ClaudeDo.App;
 
 sealed class Program
 {
+    [DllImport("shell32.dll", CharSet = CharSet.Unicode)]
+    private static extern int SetCurrentProcessExplicitAppUserModelID(string appId);
+
     [STAThread]
     public static void Main(string[] args)
     {
+        if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            SetCurrentProcessExplicitAppUserModelID("ClaudeDo.App");
+
         var services = BuildServices();
         App.Services = services;