Fix all CRITICAL and MEDIUM PCP issues

- Text domain: cs-code-block → cloudscale-code-block (header + all 70+ i18n calls)
- Echo output: inline $theme attr, wrap $cloudscale_link and $title_html with wp_kses_post(), esc_attr() on $lang_class at point of output
- MissingUnslash: add wp_unslash() before sanitize_text_field() on $_POST['theme'] and $_POST['theme_pair']

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: Andrew Baker