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/Modals/DiffModalViewModel.cs

@@ -31,8 +31,8 @@ public sealed class DiffLineViewModel
 public sealed class DiffFileViewModel
 {
     public required string Path { get; init; }
-    public int Additions { get; init; }
-    public int Deletions { get; init; }
+    public int Additions { get; set; }
+    public int Deletions { get; set; }
     public ObservableCollection<DiffLineViewModel> Lines { get; } = new();
 }
 
@@ -50,6 +50,7 @@ public sealed partial class DiffModalViewModel : ViewModelBase
     public ObservableCollection<DiffFileViewModel> Files { get; } = new();
 
     [ObservableProperty] private DiffFileViewModel? _selectedFile;
+    [ObservableProperty] private string? _statusMessage;
 
     // Injected action to close the owning Window
     public Action? CloseAction { get; set; }
@@ -79,6 +80,7 @@ public sealed partial class DiffModalViewModel : ViewModelBase
     public async Task LoadAsync(CancellationToken ct = default)
     {
         Files.Clear();
+        StatusMessage = null;
 
         string raw;
         try
@@ -87,9 +89,17 @@ public sealed partial class DiffModalViewModel : ViewModelBase
                 ? await _git.GetBranchDiffAsync(WorktreePath, BaseRef, ct)
                 : await _git.GetDiffAsync(WorktreePath, ct);
         }
-        catch { return; }
+        catch (Exception ex)
+        {
+            StatusMessage = $"Failed to load diff: {ex.Message}";
+            return;
+        }
 
-        if (string.IsNullOrWhiteSpace(raw)) return;
+        if (string.IsNullOrWhiteSpace(raw))
+        {
+            StatusMessage = "No changes to show.";
+            return;
+        }
 
         // Parse unified diff — state machine over lines
         DiffFileViewModel? current = null;
@@ -134,7 +144,7 @@ public sealed partial class DiffModalViewModel : ViewModelBase
                     NewNo = newLine++,
                     Text = line.Length > 1 ? line[1..] : "",
                 });
-                // Count additions on the file VM
+                current.Additions++;
             }
             else if (line.StartsWith('-'))
             {
@@ -144,6 +154,7 @@ public sealed partial class DiffModalViewModel : ViewModelBase
                     OldNo = oldLine++,
                     Text = line.Length > 1 ? line[1..] : "",
                 });
+                current.Deletions++;
             }
             else if (line.StartsWith(' '))
             {
@@ -158,6 +169,7 @@ public sealed partial class DiffModalViewModel : ViewModelBase
         }
 
         SelectedFile = Files.Count > 0 ? Files[0] : null;
+        if (Files.Count == 0) StatusMessage = "No changes to show.";
     }
 
     private static void ParseHunkHeader(string header, out int oldStart, out int newStart)