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.

src/ClaudeDo.Ui/Views/Modals/UsageMonitorModalView.axaml

@@ -180,6 +180,12 @@
         <TextBlock Classes="meta" VerticalAlignment="Center" Text="{Binding AnalyticsStampText}"/>
         <Button Classes="btn" Content="{loc:Tr modals.usageMonitor.analyticsRefresh}"
                 Command="{Binding RefreshAnalyticsCommand}"/>
+        <Button Classes="btn" Content="{loc:Tr modals.usageMonitor.openDashboard}"
+                ToolTip.Tip="{loc:Tr modals.usageMonitor.openDashboardTip}"
+                IsEnabled="{Binding !IsOpeningDashboard}"
+                Command="{Binding OpenDashboardCommand}"/>
+        <Ellipse Classes="spinner" Width="14" Height="14" VerticalAlignment="Center"
+                 IsVisible="{Binding IsOpeningDashboard}"/>
       </StackPanel>
 
       <TabControl Padding="20,12" TabStripPlacement="Top">