fix(desktop): keep New file/Deleted badge visible in single-file diffs

The /dev/null label fix made getDiffFileLabel return the bare file path
for new/deleted files, which now matches fallbackFilePath in single-file
diffs — so showFileHeader skipped the per-file header entirely, hiding
the New file/Deleted type badge and the +/- change counts.

Show the header whenever the diff type is notable (add/delete/rename/
copy), while still collapsing it for plain modifications where it would
only repeat the path shown in the card header.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Matt Toohey <contact@matttoohey.com>

desktop/src/features/messages/ui/DiffViewer.tsx

@@ -79,10 +79,13 @@ export function DiffViewer({
       <div className="space-y-3">
         {files.map((file) => {
           const label = getDiffFileLabel(file, fallbackFilePath);
-          const showFileHeader =
-            files.length > 1 || !fallbackFilePath || label !== fallbackFilePath;
           const { additions, deletions } = countDiffFileChanges(file);
           const diffType = normalizeDiffType(file.type);
+          const showFileHeader =
+            files.length > 1 ||
+            !fallbackFilePath ||
+            label !== fallbackFilePath ||
+            diffType !== "modify";
           const fileKey = [
             file.oldPath || "",
             file.newPath || "",