fix(i18n): add message keys for viewer, transcribe, and layout controls

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>

frontend/messages/en.json

@@ -28,6 +28,8 @@
 	"nav_conversations": "Letters",
 	"nav_admin": "Admin",
 	"nav_logout": "Sign out",
+	"layout_menu_open": "Open menu",
+	"layout_menu_close": "Close menu",
 	"theme_toggle_to_light": "Switch to light mode",
 	"theme_toggle_to_dark": "Switch to dark mode",
 	"btn_save": "Save",
@@ -394,6 +396,10 @@
 	"doc_panel_discussion_annotation_tab": "Annotation · Page {page}",
 	"pdf_annotations_show": "Show annotations",
 	"pdf_annotations_hide": "Hide annotations",
+	"viewer_previous_page": "Previous page",
+	"viewer_next_page": "Next page",
+	"viewer_zoom_out": "Zoom out",
+	"viewer_zoom_in": "Zoom in",
 	"upload_action": "Upload",
 	"upload_drop_hint": "Drop one or multiple files at once",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
@@ -655,6 +661,7 @@
 	"transcription_block_segmentation_only": "Segmentation only",
 	"training_chip_kurrent": "Kurrent recognition",
 	"training_chip_segmentation": "Segmentation",
+	"transcribe_mark_for_training": "Mark for OCR training",
 	"training_col_type": "Type",
 	"training_type_base": "Base",
 	"training_type_personalized": "Personalized",