fix: string handling for file path in markdown previews for legacy Python versions

Ensured string handling for file paths in markdown previews uses consistent single quotes to prevent syntax errors.

src/gitcloak/app.py

@@ -169,7 +169,7 @@ def preview_file(owner: str, repo: str, file_path: str):
         is_safe = False
 
         if content_type == "text/markdown":
-            base_url = f"/{owner}/{repo}/raw/main/{"/".join(file_path.split("/")[:-1])}".rstrip(
+            base_url = f"/{owner}/{repo}/raw/main/{'/'.join(file_path.split('/')[:-1])}".rstrip(
                 "/"
             )
             file_content = RelativeURLRewriter(base_url).convert(