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/DocumentBottomPanel.svelte

@@ -48,9 +48,6 @@ type Props = {
 	open: boolean;
 	height: number;
 	activeTab: Tab;
-	activeAnnotationId: string | null;
-	activeAnnotationPage: number | null;
-	onAnnotationCommentCountChange?: (annotationId: string, count: number) => void;
 };
 
 let {
@@ -61,10 +58,7 @@ let {
 	canAdmin,
 	open = $bindable(),
 	height = $bindable(),
-	activeTab = $bindable(),
-	activeAnnotationId,
-	activeAnnotationPage,
-	onAnnotationCommentCountChange
+	activeTab = $bindable()
 }: Props = $props();
 
 const MIN_HEIGHT = 52; // drag handle (8px) + tab bar (~44px)
@@ -187,13 +181,10 @@ const panelHeight = $derived(open ? height : MIN_HEIGHT);
 			{:else if activeTab === 'discussion'}
 				<PanelDiscussion
 					documentId={doc.id}
-					activeAnnotationId={activeAnnotationId}
-					activeAnnotationPage={activeAnnotationPage}
 					initialComments={comments}
 					canComment={canComment}
 					currentUserId={currentUserId}
 					canAdmin={canAdmin}
-					onAnnotationCommentCountChange={onAnnotationCommentCountChange}
 				/>
 			{:else if activeTab === 'history'}
 				<PanelHistory documentId={doc.id} />