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/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>