fix(transcription): always show comment list, compose box on demand

Comments were only visible after clicking "Kommentieren". Now:
- Comment list always renders (CommentThread with loadOnMount=true)
- Compose box hidden by default (showCompose prop on CommentThread)
- Clicking "Kommentieren" sets commentOpen=true → shows compose box
- Closing hides compose box but comments remain visible

This separates "viewing comments" (always) from "writing a comment"
(on demand via Kommentieren button).

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

frontend/src/lib/components/CommentThread.svelte

@@ -17,6 +17,7 @@ type Props = {
 	canAdmin: boolean;
 	targetCommentId?: string | null;
 	quotedText?: string | null;
+	showCompose?: boolean;
 	onCountChange?: (count: number) => void;
 };
 
@@ -31,6 +32,7 @@ let {
 	canAdmin,
 	targetCommentId = null,
 	quotedText = null,
+	showCompose = true,
 	onCountChange
 }: Props = $props();
 
@@ -357,7 +359,7 @@ onMount(async () => {
 	{/each}
 
 	<!-- New top-level comment -->
-	{#if canComment}
+	{#if canComment && showCompose}
 		<div class={comments.length > 0 ? 'border-t border-line pt-4' : ''}>
 			<div class="flex flex-col gap-2">
 				<MentionEditor