feat(frontend): move annotation comments to right-side panel

Annotation threads now open in a slide-in side panel (320 px, right
edge of the PDF viewer) instead of expanding the bottom drawer.
The PDF stays visible while the user reads and writes annotation
comments.

- Add AnnotationSidePanel component (absolute-positioned, CSS slide
  transition, keyed CommentThread, close via X or Escape)
- Remove the $effect that opened the bottom drawer on annotation click
- Simplify PanelDiscussion back to document-level thread only (no
  annotation sub-tabs)
- Remove annotation-related props from DocumentBottomPanel and
  PanelDiscussion

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

frontend/src/lib/components/PanelDiscussion.svelte

@@ -1,5 +1,4 @@
 <script lang="ts">
-import { m } from '$lib/paraglide/messages.js';
 import CommentThread from './CommentThread.svelte';
 
 type CommentReply = {
@@ -23,107 +22,21 @@ type Comment = {
 
 type Props = {
 	documentId: string;
-	activeAnnotationId: string | null;
-	activeAnnotationPage: number | null;
 	initialComments: Comment[];
 	canComment: boolean;
 	currentUserId: string | null;
 	canAdmin: boolean;
-	onAnnotationCommentCountChange?: (annotationId: string, count: number) => void;
 };
 
-let {
-	documentId,
-	activeAnnotationId,
-	activeAnnotationPage,
-	initialComments,
-	canComment,
-	currentUserId,
-	canAdmin,
-	onAnnotationCommentCountChange
-}: Props = $props();
-
-// Sub-tab within the discussion panel: 'document' or 'annotation'
-type DiscussionTab = 'document' | 'annotation';
-let activeSubTab = $state<DiscussionTab>('document');
-
-// Track document-level comment count for badge.
-// CommentThread calls onCountChange immediately on mount with the accurate total.
-let docCommentCount = $state(0);
-
-// When an annotation becomes active, switch to the annotation sub-tab automatically.
-$effect(() => {
-	if (activeAnnotationId) {
-		activeSubTab = 'annotation';
-	} else {
-		activeSubTab = 'document';
-	}
-});
-
-function selectDocumentTab() {
-	activeSubTab = 'document';
-}
-
-function selectAnnotationTab() {
-	activeSubTab = 'annotation';
-}
+let { documentId, initialComments, canComment, currentUserId, canAdmin }: Props = $props();
 </script>
 
-<div class="flex h-full flex-col">
-	<!-- Sub-tab bar (only shown when annotation is active) -->
-	{#if activeAnnotationId}
-		<div class="flex shrink-0 border-b border-brand-sand/70 bg-gray-50 px-4">
-			<button
-				onclick={selectDocumentTab}
-				class="mr-1 px-3 py-2 font-sans text-xs font-medium transition-colors {activeSubTab === 'document'
-					? 'border-b-2 border-brand-navy text-brand-navy'
-					: 'text-gray-400 hover:text-brand-navy'}"
-			>
-				{m.doc_panel_tab_discussion()}
-				{#if docCommentCount > 0}
-					<span
-						class="ml-1 inline-flex h-4 min-w-4 items-center justify-center rounded-full bg-brand-mint px-1 font-mono text-[10px] text-brand-navy"
-					>
-						{docCommentCount}
-					</span>
-				{/if}
-			</button>
-			<button
-				onclick={selectAnnotationTab}
-				class="px-3 py-2 font-sans text-xs font-medium transition-colors {activeSubTab === 'annotation'
-					? 'border-b-2 border-brand-navy text-brand-navy'
-					: 'text-gray-400 hover:text-brand-navy'}"
-			>
-				{m.doc_panel_discussion_annotation_tab({ page: String(activeAnnotationPage ?? '?') })}
-			</button>
-		</div>
-	{/if}
-
-	<!-- Content area -->
-	<div class="flex-1 overflow-y-auto p-6">
-		{#if !activeAnnotationId || activeSubTab === 'document'}
-			<!-- Document-level thread -->
-			<CommentThread
-				documentId={documentId}
-				initialComments={initialComments}
-				canComment={canComment}
-				currentUserId={currentUserId}
-				canAdmin={canAdmin}
-				onCountChange={(count) => (docCommentCount = count)}
-			/>
-		{:else}
-			<!-- Annotation-level thread -->
-			{#key activeAnnotationId}
-				<CommentThread
-					documentId={documentId}
-					annotationId={activeAnnotationId}
-					canComment={canComment}
-					currentUserId={currentUserId}
-					canAdmin={canAdmin}
-					loadOnMount={true}
-					onCountChange={(count) => onAnnotationCommentCountChange?.(activeAnnotationId, count)}
-				/>
-			{/key}
-		{/if}
-	</div>
+<div class="flex-1 overflow-y-auto p-6">
+	<CommentThread
+		documentId={documentId}
+		initialComments={initialComments}
+		canComment={canComment}
+		currentUserId={currentUserId}
+		canAdmin={canAdmin}
+	/>
 </div>