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/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'}