fix(ui): offload UnifiedDiffParser.Parse off the UI thread in DiffViewerViewModel

Both call sites (LoadFilesAsync, OnDisplayedDiffChanged) ran the parser
synchronously on the dispatcher, freezing the window on a large diff. The
Files-mode parse+tree-build now runs via Task.Run behind a ParseOp
OperationStatus wired to the toolbar; the planning-mode parse is
fire-and-forget with a generation counter so a fast second DisplayedDiff
change can't write stale PlanningFiles.

src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml

@@ -48,6 +48,7 @@
                       ToolTip.Tip="{loc:Tr modals.diff.wrapLines}">
           <PathIcon Data="{StaticResource Icon.Wrap}" Width="14" Height="14"/>
         </ToggleButton>
+        <ctl:OperationIndicator Status="{Binding ParseOp}"/>
       </StackPanel>
 
       <!-- Planning toolbar: combined-mode toggle + warning/loading -->