fix: case-sensitive username filtering causing silent backup failures

GitHub's API accepts usernames in any case but returns canonical case.
The case-sensitive comparison in filter_repositories() filtered out all
repositories when user-provided case didn't match GitHub's canonical case.

Changed to case-insensitive comparison.

Fixes #198

github_backup/github_backup.py

@@ -1587,7 +1587,9 @@ def filter_repositories(args, unfiltered_repositories):
     repositories = []
     for r in unfiltered_repositories:
         # gists can be anonymous, so need to safely check owner
-        if r.get("owner", {}).get("login") == args.user or r.get("is_starred"):
+        # Use case-insensitive comparison to match GitHub's case-insensitive username behavior
+        owner_login = r.get("owner", {}).get("login", "")
+        if owner_login.lower() == args.user.lower() or r.get("is_starred"):
             repositories.append(r)
 
     name_regex = None