default_path_for now routes posts whose lang differs from default_lang
into content_dir/<lang>/<slug>.md, matching the documented multi-language
layout and preventing same-slug posts in different languages from
overwriting each other.
- Enforce SLUG_REGEX (alphanumeric + dot/dash/underscore only) in creation_error
to prevent path traversal (C-1) and stored XSS (C-3).
- Add File.expand_path containment check in default_path_for as defense-in-depth.
- Escape slug in form action attributes (list, form views).
- Validate username in change_username using the same regex as create_user (C-4).
- Escape username in settings form action attributes.
- Treat submitting the current username as a no-op success instead of 'taken'.
petrbalvin