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)",

frontend/src/lib/components/CommentThread.svelte

@@ -196,24 +196,15 @@ onMount(() => {
 	</div>
 {/if}
 
-{#if canComment && showCompose}
-	<div class="mt-2 flex gap-2">
-		<div class="flex-1">
-			<MentionEditor
-				bind:value={newText}
-				bind:mentionCandidates={newMentionCandidates}
-				rows={1}
-				placeholder={m.comment_placeholder()}
-				disabled={posting}
-				onsubmit={postComment}
-			/>
-		</div>
-		<button
-			class="self-end rounded bg-primary px-2.5 py-1.5 font-sans text-xs font-medium text-primary-fg hover:bg-primary/80 disabled:opacity-40"
-			disabled={posting || !newText.trim()}
-			onclick={postComment}
-		>
-			{m.comment_btn_post()}
-		</button>
+{#if canComment && (showCompose || flatMessages.length > 0)}
+	<div class="mt-2">
+		<MentionEditor
+			bind:value={newText}
+			bind:mentionCandidates={newMentionCandidates}
+			rows={1}
+			placeholder={m.comment_placeholder()}
+			disabled={posting}
+			onsubmit={postComment}
+		/>
 	</div>
 {/if}

frontend/src/lib/components/MentionEditor.svelte

@@ -115,7 +115,8 @@ function closePopup() {
 }
 
 function handleKeydown(e: KeyboardEvent) {
-	if (e.ctrlKey && e.key === 'Enter') {
+	// Enter sends, Shift+Enter adds newline
+	if (e.key === 'Enter' && !e.shiftKey && query === null) {
 		e.preventDefault();
 		onsubmit?.();
 		return;
@@ -152,33 +153,6 @@ function handleKeydown(e: KeyboardEvent) {
 	}
 }
 
-async function handleAtButtonClick() {
-	if (!textarea) return;
-	const pos = textarea.selectionStart;
-	const before = value.slice(0, pos);
-	const after = value.slice(pos);
-	// Ensure @ is preceded by whitespace or is at the start
-	const needsSpace = before.length > 0 && !/\s$/.test(before);
-	const insertion = needsSpace ? ' @' : '@';
-	value = before + insertion + after;
-
-	await tick();
-	if (!textarea) return;
-	const newPos = pos + insertion.length;
-	textarea.selectionStart = newPos;
-	textarea.selectionEnd = newPos;
-	textarea.focus();
-
-	// Trigger mention detection after inserting @
-	const detected = detectMention(value, newPos);
-	if (detected !== null) {
-		mentionStart = newPos - 1;
-		query = detected;
-		highlightedIndex = 0;
-		scheduleSearch(detected);
-	}
-}
-
 onDestroy(() => clearTimeout(debounceTimer));
 
 const popupOpen = $derived(query !== null);
@@ -224,14 +198,4 @@ const popupOpen = $derived(query !== null);
 			{/if}
 		</div>
 	{/if}
-
-	<button
-		type="button"
-		aria-label={m.mention_btn_label()}
-		disabled={disabled}
-		class="mt-1 rounded border border-line px-2 py-0.5 font-sans text-xs font-medium text-ink-3 transition-colors hover:border-ink hover:text-ink disabled:opacity-40"
-		onclick={handleAtButtonClick}
-	>
-		@
-	</button>
 </div>

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}