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

@@ -3,6 +3,7 @@ import { onMount } from 'svelte';
 import DocumentTopBar from '$lib/components/DocumentTopBar.svelte';
 import DocumentViewer from '$lib/components/DocumentViewer.svelte';
 import DocumentBottomPanel from '$lib/components/DocumentBottomPanel.svelte';
+import AnnotationSidePanel from '$lib/components/AnnotationSidePanel.svelte';
 
 type Tab = 'metadata' | 'transcription' | 'discussion' | 'history';
 
@@ -58,14 +59,6 @@ let annotateMode = $state(false);
 let activeAnnotationId = $state<string | null>(null);
 let activeAnnotationPage = $state<number | null>(null);
 
-// When an annotation is clicked, open the Diskussion tab.
-$effect(() => {
-	if (activeAnnotationId) {
-		activeTab = 'discussion';
-		panelOpen = true;
-	}
-});
-
 // Close the panel when entering annotate mode so the PDF is fully visible.
 $effect(() => {
 	if (annotateMode) panelOpen = false;
@@ -152,6 +145,18 @@ $effect(() => {
 				activeAnnotationId = id;
 			}}
 		/>
+		<AnnotationSidePanel
+			documentId={doc.id}
+			activeAnnotationId={activeAnnotationId}
+			activeAnnotationPage={activeAnnotationPage}
+			canComment={canComment}
+			currentUserId={currentUserId}
+			canAdmin={canAdmin}
+			onClose={() => {
+				activeAnnotationId = null;
+				activeAnnotationPage = null;
+			}}
+		/>
 	</div>
 
 	<DocumentBottomPanel
@@ -163,7 +168,5 @@ $effect(() => {
 		bind:open={panelOpen}
 		bind:height={panelHeight}
 		bind:activeTab={activeTab}
-		activeAnnotationId={activeAnnotationId}
-		activeAnnotationPage={activeAnnotationPage}
 	/>
 </div>