refactor(diff): render planning mode per file and retire DiffLinesView

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

@@ -56,7 +56,8 @@ public sealed partial class DiffViewerViewModel : ViewModelBase
 
     // ── Right pane ──────────────────────────────────────────────────────────
     [ObservableProperty] private DiffFileViewModel? _selectedFile;       // Files mode
-    public ObservableCollection<DiffLineViewModel> DiffLines { get; } = new(); // Planning mode
+    // Planning mode: one entry per file so each gets its own editor and grammar.
+    public ObservableCollection<DiffFileViewModel> PlanningFiles { get; } = new();
     [ObservableProperty] private string _displayedDiff = "";
     [ObservableProperty] private string? _statusMessage;
 
@@ -250,9 +251,9 @@ public sealed partial class DiffViewerViewModel : ViewModelBase
 
     partial void OnDisplayedDiffChanged(string value)
     {
-        DiffLines.Clear();
-        foreach (var line in UnifiedDiffParser.Flatten(UnifiedDiffParser.Parse(value)))
-            DiffLines.Add(line);
+        PlanningFiles.Clear();
+        foreach (var file in UnifiedDiffParser.Parse(value))
+            PlanningFiles.Add(file);
     }
 
     // ── Merge (Files mode, branch source) ───────────────────────────────────