Adds 7 Paraglide keys (viewer_previous_page, viewer_next_page, viewer_zoom_out, viewer_zoom_in, transcribe_mark_for_training, layout_menu_open, layout_menu_close) to de/en/es.json. Adds messages.spec.ts to enforce key parity across all three locales. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>