feat(#73): deep-link to specific comments via ?commentId= query param

- +page.svelte: read ?commentId= from URL; on mount, if present open bottom panel to discussion tab
- CommentThread: add targetCommentId prop — scrolls to comment on mount (scrollIntoView), applies ring highlight, removes highlight on first user interaction (click/keydown/scroll)
- CommentThread: add data-comment-id attributes to thread root and reply divs
- PanelDiscussion / DocumentBottomPanel: thread targetCommentId prop through the chain

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

frontend/src/lib/components/PanelDiscussion.svelte

@@ -8,11 +8,19 @@ type Props = {
 	canComment: boolean;
 	currentUserId: string | null;
 	canAdmin: boolean;
+	targetCommentId?: string | null;
 	onCountChange?: (count: number) => void;
 };
 
-let { documentId, initialComments, canComment, currentUserId, canAdmin, onCountChange }: Props =
-	$props();
+let {
+	documentId,
+	initialComments,
+	canComment,
+	currentUserId,
+	canAdmin,
+	targetCommentId = null,
+	onCountChange
+}: Props = $props();
 </script>
 
 <div class="flex-1 overflow-y-auto p-6">
@@ -22,6 +30,7 @@ let { documentId, initialComments, canComment, currentUserId, canAdmin, onCountC
 		canComment={canComment}
 		currentUserId={currentUserId}
 		canAdmin={canAdmin}
+		targetCommentId={targetCommentId}
 		onCountChange={onCountChange}
 	/>
 </div>