feat: Make directories stand out more in local file browser: "[dir]" instead of "dir" (#1339)

## Summary

* **What is the goal of this PR?** It's difficult to distinguish
directory names from normal file entries, so they are displayed now as
"[dir]" instead of "dir" for classic theme
* **What changes are included?**

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).

---

### AI Usage

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Did you use AI tools to help write this code? _**NO**_

src/activities/home/FileBrowserActivity.cpp

@@ -239,7 +239,11 @@ void FileBrowserActivity::loop() {
 
 std::string getFileName(std::string filename) {
   if (filename.back() == '/') {
-    return filename.substr(0, filename.length() - 1);
+    filename.pop_back();
+    if (!UITheme::getInstance().getTheme().showsFileIcons()) {
+      return "[" + filename + "]";
+    }
+    return filename;
   }
   const auto pos = filename.rfind('.');
   return filename.substr(0, pos);