feat(ui): show status messages and real diff-stats in DiffModal

- Count additions/deletions per file as lines are parsed.
- Surface load failures and empty-diff states via StatusMessage.
- Pass the worktree base commit so diffs render against the branch
  base, not just the working-tree HEAD.

src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs

@@ -262,6 +262,7 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase
             Notes = entity.Notes ?? "";
             Model = entity.Model;
             WorktreePath = entity.Worktree?.Path;
+            WorktreeBaseCommit = entity.Worktree?.BaseCommit;
             WorktreeStateLabel = entity.Worktree?.State.ToString();
             BranchLine = entity.Worktree is { } w ? $"{w.BranchName} \u2190 main" : null;
             AgentStatusLabel = entity.Status.ToString();