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/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}