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/AnnotationSidePanel.svelte

@@ -0,0 +1,65 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+import CommentThread from './CommentThread.svelte';
+
+type Props = {
+	documentId: string;
+	activeAnnotationId: string | null;
+	activeAnnotationPage: number | null;
+	canComment: boolean;
+	currentUserId: string | null;
+	canAdmin: boolean;
+	onClose: () => void;
+};
+
+let {
+	documentId,
+	activeAnnotationId,
+	activeAnnotationPage,
+	canComment,
+	currentUserId,
+	canAdmin,
+	onClose
+}: Props = $props();
+
+const visible = $derived(activeAnnotationId !== null);
+</script>
+
+<div
+	class="pointer-events-none absolute inset-y-0 right-0 z-10 flex w-80 flex-col border-l border-brand-sand bg-white shadow-[-4px_0_16px_rgba(0,0,0,0.08)] transition-transform duration-200 {visible
+		? 'pointer-events-auto translate-x-0'
+		: 'translate-x-full'}"
+	data-testid="annotation-side-panel"
+>
+	<!-- Header -->
+	<div class="flex shrink-0 items-center justify-between border-b border-brand-sand px-4 py-3">
+		<span class="font-sans text-xs font-medium text-brand-navy">
+			{m.doc_panel_discussion_annotation_tab({ page: String(activeAnnotationPage ?? '?') })}
+		</span>
+		<button
+			onclick={onClose}
+			aria-label={m.comment_panel_close()}
+			class="rounded p-1 text-gray-400 transition-colors hover:bg-brand-sand/50 hover:text-brand-navy"
+		>
+			<svg class="h-4 w-4" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2">
+				<path stroke-linecap="round" stroke-linejoin="round" d="M6 18L18 6M6 6l12 12" />
+			</svg>
+		</button>
+	</div>
+
+	<!-- Comment thread -->
+	<div class="flex-1 overflow-y-auto p-4">
+		{#if activeAnnotationId}
+			{#key activeAnnotationId}
+				<CommentThread
+					documentId={documentId}
+					annotationId={activeAnnotationId}
+					canComment={canComment}
+					currentUserId={currentUserId}
+					canAdmin={canAdmin}
+					loadOnMount={true}
+				/>
+			{/key}
+		{/if}
+	</div>
+</div>