feat(merge): diff Merge opens the 3-pane editor + conflict overview ruler

- The Merge button in the Diff window now hands a conflicting merge to the in-app
  3-pane editor (MergeModal routes 'conflict' through RequestConflictResolution,
  the same seam Approve uses) instead of dead-ending on a conflict message.
- Add a conflict overview ruler right of the Result pane: a proportional map of
  every conflict in the file, recolored by resolved state, click a tick to jump —
  so conflicts are findable in long files without scrolling.
- New MergeResolvedEdgeBrush token + conflictMap en/de key. Ui 128 + Loc 16 green.

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

@@ -156,6 +156,7 @@ public sealed partial class MergeSectionViewModel : ViewModelBase
                 TaskTitle = TaskTitle ?? "",
                 ShowMergeModal = ShowMergeModal,
                 ResolveMergeVm = () => _services.GetRequiredService<MergeModalViewModel>(),
+                RequestConflictResolution = RequestConflictResolution,
             };
         }
         else if (CanDiffMergedRange)