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/de.json

@@ -28,6 +28,8 @@
 	"nav_conversations": "Briefwechsel",
 	"nav_admin": "Admin",
 	"nav_logout": "Abmelden",
+	"layout_menu_open": "Menü öffnen",
+	"layout_menu_close": "Menü schließen",
 	"theme_toggle_to_light": "Zu hellem Design wechseln",
 	"theme_toggle_to_dark": "Zu dunklem Design wechseln",
 	"btn_save": "Speichern",
@@ -394,6 +396,10 @@
 	"doc_panel_discussion_annotation_tab": "Annotation · Seite {page}",
 	"pdf_annotations_show": "Annotierungen anzeigen",
 	"pdf_annotations_hide": "Annotierungen verbergen",
+	"viewer_previous_page": "Zurück",
+	"viewer_next_page": "Weiter",
+	"viewer_zoom_out": "Verkleinern",
+	"viewer_zoom_in": "Vergrößern",
 	"upload_action": "Hochladen",
 	"upload_drop_hint": "Einzeln oder mehrere Dateien auf einmal hochladen",
 	"upload_accepted_types": "PDF, JPEG, PNG, TIFF",
@@ -655,6 +661,7 @@
 	"transcription_block_segmentation_only": "Nur Segmentierung",
 	"training_chip_kurrent": "Kurrent-Erkennung",
 	"training_chip_segmentation": "Segmentierung",
+	"transcribe_mark_for_training": "Für Training vormerken",
 	"training_col_type": "Typ",
 	"training_type_base": "Basis",
 	"training_type_personalized": "Personalisiert",