feat(ui): Empty States für Detail-Pane und Task-Liste (UX-Audit #4)

Zentrierter, gedimmter Platzhalter (Mission-Control-Muster) im
Detail-Pane bei fehlender Selektion (nicht in Notes-/Prep-Modus) und
in der Task-Liste bei 0 sichtbaren Tasks; User-Listen ohne
WorkingDir bekommen zusätzlich den Repo-Verknüpfungs-Hinweis.

tests/ClaudeDo.Ui.Tests/ViewModels/DetailsIslandEmptyStateTests.cs

@@ -0,0 +1,116 @@
+using ClaudeDo.Data;
+using ClaudeDo.Ui.Services;
+using ClaudeDo.Ui.ViewModels.Islands;
+using Microsoft.EntityFrameworkCore;
+
+namespace ClaudeDo.Ui.Tests.ViewModels;
+
+public class DetailsIslandEmptyStateTests : IDisposable
+{
+    private readonly string _dbPath;
+
+    public DetailsIslandEmptyStateTests()
+    {
+        _dbPath = Path.Combine(Path.GetTempPath(), $"claudedo_emptystate_test_{Guid.NewGuid():N}.db");
+        using var ctx = NewContext();
+        ctx.Database.EnsureCreated();
+    }
+
+    public void Dispose()
+    {
+        try { File.Delete(_dbPath); } catch { }
+        try { File.Delete(_dbPath + "-wal"); } catch { }
+        try { File.Delete(_dbPath + "-shm"); } catch { }
+    }
+
+    private ClaudeDoDbContext NewContext()
+    {
+        var opts = new DbContextOptionsBuilder<ClaudeDoDbContext>()
+            .UseSqlite($"Data Source={_dbPath}")
+            .Options;
+        return new ClaudeDoDbContext(opts);
+    }
+
+    private sealed class TestDbFactory : IDbContextFactory<ClaudeDoDbContext>
+    {
+        private readonly Func<ClaudeDoDbContext> _create;
+        public TestDbFactory(Func<ClaudeDoDbContext> create) => _create = create;
+        public ClaudeDoDbContext CreateDbContext() => _create();
+    }
+
+    private sealed class DefaultStub : StubWorkerClient { }
+
+    private sealed class StubNotesApi : ClaudeDo.Ui.Services.Interfaces.INotesApi
+    {
+        public Task<List<DailyNoteDto>> ListAsync(DateOnly day) => Task.FromResult(new List<DailyNoteDto>());
+        public Task<DailyNoteDto?> AddAsync(DateOnly day, string text) => Task.FromResult<DailyNoteDto?>(null);
+        public Task UpdateAsync(string id, string text) => Task.CompletedTask;
+        public Task DeleteAsync(string id) => Task.CompletedTask;
+    }
+
+    private sealed class NullServiceProvider : IServiceProvider
+    {
+        public object? GetService(Type serviceType) => null;
+    }
+
+    private DetailsIslandViewModel NewDetailsVm(StubWorkerClient stub)
+    {
+        var factory = new TestDbFactory(NewContext);
+        return new DetailsIslandViewModel(factory, stub, new NullServiceProvider(), new StubNotesApi(), new ClaudeDo.Ui.Services.MergeCoordinator());
+    }
+
+    [Fact]
+    public void IsEmptyStateVisible_true_when_no_task_selected()
+    {
+        var vm = NewDetailsVm(new DefaultStub());
+
+        Assert.True(vm.IsEmptyStateVisible);
+    }
+
+    [Fact]
+    public void IsEmptyStateVisible_false_once_a_task_is_bound()
+    {
+        var vm = NewDetailsVm(new DefaultStub());
+        var row = new TaskRowViewModel { Id = "t1", Title = "Task" };
+
+        vm.Bind(row, "test");
+
+        Assert.False(vm.IsEmptyStateVisible);
+    }
+
+    [Fact]
+    public void IsEmptyStateVisible_false_in_notes_mode_even_without_a_task()
+    {
+        var vm = NewDetailsVm(new DefaultStub());
+
+        vm.ShowNotes();
+
+        Assert.True(vm.IsNotesMode);
+        Assert.Null(vm.Task);
+        Assert.False(vm.IsEmptyStateVisible);
+    }
+
+    [Fact]
+    public void IsEmptyStateVisible_false_in_prep_mode_even_without_a_task()
+    {
+        var vm = NewDetailsVm(new DefaultStub());
+
+        vm.ShowPrep();
+
+        Assert.True(vm.IsPrepMode);
+        Assert.Null(vm.Task);
+        Assert.False(vm.IsEmptyStateVisible);
+    }
+
+    [Fact]
+    public void IsEmptyStateVisible_true_again_after_unbinding_a_task()
+    {
+        var vm = NewDetailsVm(new DefaultStub());
+        var row = new TaskRowViewModel { Id = "t1", Title = "Task" };
+        vm.Bind(row, "test");
+
+        vm.Bind(null, "test");
+
+        Assert.True(vm.IsEmptyStateVisible);
+    }
+}