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/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}