feat(usage): open the TokenTracker dashboard from the usage monitor

The modal deliberately shows only a slice of the analytics; this hands off
to TokenTracker's own local dashboard for the rest. The worker starts
`tokentracker serve` on demand and returns the URL, the UI opens the browser.

Three things the spawn has to get right: port 7680 is not free on Windows
(Delivery Optimization holds [::]:7680) and serve does not fall back, so we
scan 7680-7689 with a dual-stack bind probe; --no-open because the CLI would
open the browser before the server answers; and the child is a cmd.exe shim,
so shutdown kills the process tree. --no-sync keeps our no-cloud-sync rule.

tests/ClaudeDo.Worker.Tests/Usage/TokenTracker/FakeTokenTrackerClient.cs

@@ -38,6 +38,15 @@ public sealed class FakeTokenTrackerClient : ITokenTrackerClient
         return ExportResult;
     }
 
+    public TokenTrackerDashboard DashboardResult { get; set; } = new(true, "http://127.0.0.1:7680/", null);
+    public int DashboardCalls;
+
+    public Task<TokenTrackerDashboard> StartDashboardAsync(CancellationToken ct = default)
+    {
+        Interlocked.Increment(ref DashboardCalls);
+        return Task.FromResult(DashboardResult);
+    }
+
     public Task<TokenTrackerRunResult> InstallAsync(IProgress<string>? output = null, CancellationToken ct = default)
     {
         Interlocked.Increment(ref InstallCalls);