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.

src/ClaudeDo.Ui/ViewModels/Modals/SettingsModalViewModel.cs

@@ -56,6 +56,8 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
 
     // True once at least one list has a linked repo (WorkingDir). Drives the repo-hint strip on
     // Worktrees/Prime Claude/Session Skills/Berichte — nothing is ever hidden, just annotated.
+    // Scope intentionally broader than ListsIslandViewModel.HasNoLinkedRepo (all Lists rows in
+    // the DB, not just UserLists).
     [ObservableProperty] private bool _hasLinkedRepo;
 
     // Wired by WindowDialogService to close this modal and open the existing repo-import flow.
@@ -152,7 +154,7 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
             General.LoadModelPresets(dto?.ModelPresets, General.MaxTurnsCeiling);
 
             await using var ctx = await _dbFactory.CreateDbContextAsync();
-            HasLinkedRepo = await ctx.Lists.AnyAsync(l => l.WorkingDir != null && l.WorkingDir != "");
+            HasLinkedRepo = await ctx.Lists.AnyAsync(RepoLinkage.IsLinkedInDb);
         }
         finally { IsBusy = false; }
     }