fix: Sort root-level files to bottom of sidebar file tree

Matches editor convention where directory-grouped files appear first
and loose root-level files (e.g., globals.css, pnpm-lock.yaml) sort last.

apps/web/src/components/review/ReviewSidebar.tsx

@@ -138,9 +138,13 @@ function groupFilesByDirectory(files: FileDiff[]): DirectoryGroup[] {
     }
   }
 
-  // Sort directories alphabetically, sort files within each directory
+  // Sort directories alphabetically, root-level files ("") last
   const sorted = Array.from(groups.entries())
-    .sort(([a], [b]) => a.localeCompare(b))
+    .sort(([a], [b]) => {
+      if (a === "" && b !== "") return 1;
+      if (b === "" && a !== "") return -1;
+      return a.localeCompare(b);
+    })
     .map(([directory, dirFiles]) => ({
       directory,
       files: dirFiles.sort((a, b) => {