feat(worker): run worker as per-user logon task instead of Windows service

A LocalSystem Windows service can't see the logged-in user's Claude CLI
authentication, so the worker now runs as the current user via a hidden
per-user logon Scheduled Task with restart-on-failure.

- Worker is WinExe (no console window) with a Serilog rolling file sink and
  a single-instance mutex so the logon task, app ensure-running, and Restart
  button can't fight over the SignalR port.
- Installer replaces the service steps (register/start/stop) with autostart
  task steps, migrates the legacy ClaudeDoWorker service away on update, and
  removes the task on uninstall. ServicePage drops the service-account UI.
- UI gains a WorkerLocator; the app ensures the worker is running at startup
  and the Restart button kills+relaunches this install's worker process.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Installer/Steps/StartWorkerStep.cs

@@ -0,0 +1,19 @@
+using ClaudeDo.Installer.Core;
+
+namespace ClaudeDo.Installer.Steps;
+
+public sealed class StartWorkerStep : IInstallStep
+{
+    public const string TaskName = "ClaudeDoWorker";
+
+    public string Name => "Start Worker";
+
+    public async Task<StepResult> ExecuteAsync(InstallContext ctx, IProgress<string> progress, CancellationToken ct)
+    {
+        progress.Report("Starting worker...");
+        var (exit, output) = await ProcessRunner.RunAsync("schtasks.exe", $"/Run /TN \"{TaskName}\"", null, progress, ct);
+        if (exit != 0)
+            return StepResult.Fail($"schtasks /Run failed (exit {exit}): {output}");
+        return StepResult.Ok();
+    }
+}