feat(frontend): add annotation visibility toggle to PDF toolbar

Eye/eye-slash button in the PDF controls bar lets the user hide all
annotation highlights to read the document unobstructed and show them again
with one click.

- Button only renders when at least one annotation exists
- Active state (hidden) highlighted with brand-mint/bg-white/10 so the
  current state is always clear
- i18n keys added for de/en/es

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/es.json

@@ -258,5 +258,7 @@
 	"doc_panel_annotate": "Anotar",
 	"doc_panel_annotate_stop": "Listo",
 	"doc_panel_annotation_thread_title": "Anotación",
-	"doc_panel_discussion_annotation_tab": "Anotación · Página {page}"
+	"doc_panel_discussion_annotation_tab": "Anotación · Página {page}",
+	"pdf_annotations_show": "Mostrar anotaciones",
+	"pdf_annotations_hide": "Ocultar anotaciones"
 }