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/src/lib/components/TranscriptionBlock.svelte

@@ -196,7 +196,7 @@ function captureSelectionAndOpenComments() {
 		</div>
 
 		<!-- Comment thread — list always visible, compose toggled by Kommentieren -->
-		<div class="mt-3 border-t border-line pt-3">
+		<div class="mt-3">
 			<CommentThread
 				documentId={documentId}
 				blockId={blockId}