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/CommentThread.svelte

@@ -14,6 +14,7 @@ type Props = {
 	canComment: boolean;
 	currentUserId: string | null;
 	canAdmin: boolean;
+	targetCommentId?: string | null;
 	onCountChange?: (count: number) => void;
 };
 
@@ -25,10 +26,12 @@ let {
 	canComment,
 	currentUserId,
 	canAdmin,
+	targetCommentId = null,
 	onCountChange
 }: Props = $props();
 
 let comments: Comment[] = $state(untrack(() => [...initialComments]));
+let highlightedCommentId: string | null = $state(untrack(() => targetCommentId ?? null));
 let newText: string = $state('');
 let replyingTo: string | null = $state(null);
 let replyText: string = $state('');
@@ -184,6 +187,25 @@ onMount(() => {
 		const total = initialComments.reduce((s, c) => s + 1 + c.replies.length, 0);
 		onCountChange?.(total);
 	}
+
+	if (targetCommentId) {
+		// Scroll to target after a tick so the DOM is settled
+		setTimeout(() => {
+			const el = document.querySelector(`[data-comment-id="${targetCommentId}"]`);
+			el?.scrollIntoView({ behavior: 'smooth', block: 'center' });
+		}, 100);
+
+		// Remove highlight on first user interaction
+		const clearHighlight = () => {
+			highlightedCommentId = null;
+			document.removeEventListener('click', clearHighlight, true);
+			document.removeEventListener('keydown', clearHighlight, true);
+			document.removeEventListener('scroll', clearHighlight, true);
+		};
+		document.addEventListener('click', clearHighlight, true);
+		document.addEventListener('keydown', clearHighlight, true);
+		document.addEventListener('scroll', clearHighlight, true);
+	}
 });
 </script>
 
@@ -287,13 +309,23 @@ onMount(() => {
 	{#each comments as thread, ti (thread.id)}
 		<div class={ti > 0 ? 'border-t border-line pt-4' : ''}>
 			<!-- Root comment -->
-			<div>
+			<div
+				data-comment-id={thread.id}
+				class={highlightedCommentId === thread.id
+					? 'rounded ring-2 ring-accent ring-offset-1 transition-shadow'
+					: ''}
+			>
 				{@render commentEntry(thread, thread.id, thread.replies.length === 0)}
 			</div>
 
 			<!-- Replies -->
 			{#each thread.replies as reply, ri (reply.id)}
-				<div class="mt-3 ml-6 border-l-2 border-line pl-4">
+				<div
+					data-comment-id={reply.id}
+					class="mt-3 ml-6 border-l-2 border-line pl-4 {highlightedCommentId === reply.id
+						? 'rounded ring-2 ring-accent ring-offset-1 transition-shadow'
+						: ''}"
+				>
 					{@render commentEntry(reply, thread.id, ri === thread.replies.length - 1)}
 				</div>
 			{/each}

frontend/src/lib/components/DocumentBottomPanel.svelte

@@ -28,6 +28,7 @@ type Props = {
 	open: boolean;
 	height: number;
 	activeTab: DocumentPanelTab;
+	targetCommentId?: string | null;
 };
 
 let {
@@ -38,7 +39,8 @@ let {
 	canAdmin,
 	open = $bindable(),
 	height = $bindable(),
-	activeTab = $bindable()
+	activeTab = $bindable(),
+	targetCommentId = null
 }: Props = $props();
 
 const MIN_HEIGHT = 52; // drag handle (8px) + tab bar (~44px)
@@ -180,6 +182,7 @@ function handleCountChange(count: number) {
 					canComment={canComment}
 					currentUserId={currentUserId}
 					canAdmin={canAdmin}
+					targetCommentId={targetCommentId}
 					onCountChange={handleCountChange}
 				/>
 			{:else if activeTab === 'history'}

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>

frontend/src/routes/documents/[id]/+page.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 import { onMount } from 'svelte';
+import { page } from '$app/state';
 import DocumentTopBar from '$lib/components/DocumentTopBar.svelte';
 import DocumentViewer from '$lib/components/DocumentViewer.svelte';
 import DocumentBottomPanel from '$lib/components/DocumentBottomPanel.svelte';
@@ -8,6 +9,8 @@ import type { DocumentPanelTab } from '$lib/types';
 
 let { data } = $props();
 
+const targetCommentId = $derived(page.url.searchParams.get('commentId'));
+
 const doc = $derived(data.document);
 const canComment = $derived((data.canAnnotate || data.canWrite) ?? false);
 const canAdmin = $derived(
@@ -92,7 +95,11 @@ onMount(() => {
 		if (!isNaN(h) && h >= 80) panelHeight = h;
 	}
 
-	if (savedOpen === 'true') {
+	if (targetCommentId) {
+		// Deep-link: always open discussion tab regardless of saved state
+		panelOpen = true;
+		activeTab = 'discussion';
+	} else if (savedOpen === 'true') {
 		panelOpen = true;
 	} else if (savedOpen === null && !doc?.filePath) {
 		// No prior state and no file — open to metadata so the panel is immediately useful.
@@ -175,6 +182,7 @@ $effect(() => {
 		canComment={canComment}
 		currentUserId={currentUserId}
 		canAdmin={canAdmin}
+		targetCommentId={targetCommentId}
 		bind:open={panelOpen}
 		bind:height={panelHeight}
 		bind:activeTab={activeTab}