refactor(comments): streamline input — Enter to send, no buttons

- MentionEditor: Enter sends (Shift+Enter for newline), remove @ button
- CommentThread: remove send button, full-width input, always show
  input when comments exist (no need to click Kommentieren first)
- TranscriptionBlock: remove border-t above comment section (orange
  background provides enough visual separation)
- Update placeholder in all languages to hint @mention and Enter to send

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

frontend/messages/de.json

@@ -315,7 +315,7 @@
 	"comp_expandable_show_less": "Weniger anzeigen",
 	"error_comment_not_found": "Der Kommentar wurde nicht gefunden.",
 	"comment_section_title": "Diskussion",
-	"comment_placeholder": "Kommentar schreiben…",
+	"comment_placeholder": "Kommentar schreiben… (@Name erwähnen · Enter senden)",
 	"comment_btn_post": "Senden",
 	"comment_btn_reply": "Antworten",
 	"comment_edited_label": "(Bearbeitet)",

frontend/messages/en.json

@@ -315,7 +315,7 @@
 	"comp_expandable_show_less": "Show less",
 	"error_comment_not_found": "The comment could not be found.",
 	"comment_section_title": "Discussion",
-	"comment_placeholder": "Write a comment…",
+	"comment_placeholder": "Write a comment… (@name to mention · Enter to send)",
 	"comment_btn_post": "Send",
 	"comment_btn_reply": "Reply",
 	"comment_edited_label": "(Edited)",

frontend/messages/es.json

@@ -315,7 +315,7 @@
 	"comp_expandable_show_less": "Mostrar menos",
 	"error_comment_not_found": "El comentario no pudo encontrarse.",
 	"comment_section_title": "Discusión",
-	"comment_placeholder": "Escribe un comentario…",
+	"comment_placeholder": "Escribe un comentario… (@nombre para mencionar · Enter para enviar)",
 	"comment_btn_post": "Enviar",
 	"comment_btn_reply": "Responder",
 	"comment_edited_label": "(Editado)",