refactor(ui): unify list-linked-repo predicate into RepoLinkage

ListsIslandViewModel and SettingsModalViewModel each re-implemented "does
this list have a linked WorkingDir" with different whitespace handling.
RepoLinkage.IsLinked/IsLinkedInDb is now the single definition; both
callers derive from it, closing the whitespace-only WorkingDir gap where
the Settings modal disagreed with the ListsIsland banner.

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

@@ -167,6 +167,21 @@ public class SettingsModalViewModelTests : IDisposable
         Assert.True(vm.HasLinkedRepo);
     }
 
+    [Fact]
+    public async Task HasLinkedRepo_false_when_a_list_working_dir_is_whitespace_only()
+    {
+        using (var ctx = NewContext())
+        {
+            ctx.Lists.Add(new ListEntity { Id = "l1", Name = "Blank repo", CreatedAt = DateTime.UtcNow, WorkingDir = "   " });
+            ctx.SaveChanges();
+        }
+        var vm = MakeVm(new FakeWorker());
+
+        await vm.LoadAsync();
+
+        Assert.False(vm.HasLinkedRepo);
+    }
+
     [Fact]
     public void SelectedCategory_updates_SelectedIndex_and_ignores_null()
     {