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.Localization/locales/en.json

@@ -698,6 +698,9 @@
       "merging": "Merging…",
       "verifying": "Running the list's verify command… ({0})"
     },
+    "diff": {
+      "parsing": "Parsing diff…"
+    },
     "review": {
       "submitting": "Submitting for review…",
       "rejecting": "Rejecting…",