feat(ui): prompt once on worker connection failure with grace timer

Adds ShowWorkerConnectionModal hook, DecideShowConnectionPrompt one-shot gate, OpenWorkerConnectionHelp relay command, and a 12 s _connectTimer to IslandsShellViewModel; covered by two new unit tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/ClaudeDo.Ui.Tests/ConnectionPromptGateTests.cs

@@ -0,0 +1,22 @@
+using ClaudeDo.Ui.ViewModels;
+using Xunit;
+
+namespace ClaudeDo.Ui.Tests;
+
+public class ConnectionPromptGateTests
+{
+    [Fact]
+    public void Shows_once_when_offline()
+    {
+        var vm = new IslandsShellViewModel();
+        Assert.True(vm.DecideShowConnectionPrompt(isOffline: true));
+        Assert.False(vm.DecideShowConnectionPrompt(isOffline: true)); // not a second time
+    }
+
+    [Fact]
+    public void Does_not_show_when_connected_before_grace()
+    {
+        var vm = new IslandsShellViewModel();
+        Assert.False(vm.DecideShowConnectionPrompt(isOffline: false));
+    }
+}