feat(#81): improve discussion discoverability

- Add comment count badge on the Discussion tab (seeded from SSR, updated live)
- Add 'Diskussion starten' nudge above collapsed panel when no comments exist
- Add empty state hint with speech-bubble icon inside the discussion panel
- Fix CommentThread to fire onCountChange with SSR-seeded count on mount
- Add tests for all three behaviours in CommentThread and DocumentBottomPanel

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

frontend/messages/de.json

@@ -291,5 +291,7 @@
 	"enrich_skip": "Überspringen",
 	"enrich_done_heading": "Alles erledigt!",
 	"enrich_done_body": "Alle Dokumente wurden bearbeitet.",
-	"enrich_back_to_list": "Zurück zur Liste"
+	"enrich_back_to_list": "Zurück zur Liste",
+	"comment_empty_hint": "Noch keine Kommentare – starte die Diskussion!",
+	"comment_start_discussion": "Diskussion starten →"
 }

frontend/messages/en.json

@@ -291,5 +291,7 @@
 	"enrich_skip": "Skip",
 	"enrich_done_heading": "All done!",
 	"enrich_done_body": "All documents have been processed.",
-	"enrich_back_to_list": "Back to list"
+	"enrich_back_to_list": "Back to list",
+	"comment_empty_hint": "No comments yet – start the discussion!",
+	"comment_start_discussion": "Start discussion →"
 }

frontend/messages/es.json

@@ -291,5 +291,7 @@
 	"enrich_skip": "Omitir",
 	"enrich_done_heading": "¡Todo listo!",
 	"enrich_done_body": "Todos los documentos han sido procesados.",
-	"enrich_back_to_list": "Volver a la lista"
+	"enrich_back_to_list": "Volver a la lista",
+	"comment_empty_hint": "Aún no hay comentarios – ¡inicia la discusión!",
+	"comment_start_discussion": "Iniciar discusión →"
 }