feat(frontend): add floating bottom panel to document detail page

Replaces the left sidebar layout with:
- Full-viewport PDF/image viewer (never resizes, position: absolute)
- Fixed floating bottom panel with tabs: Metadaten, Transkription,
  Diskussion, Verlauf
- Compact top bar with title, date · sender → receivers row, and
  Annotieren / Edit / Download actions
- Drag-to-resize panel with localStorage persistence of open/height/tab
- Panel opens automatically to Diskussion when an annotation is clicked
- Documents without a file default to showing the Metadaten tab

New components: DocumentTopBar, DocumentViewer, DocumentBottomPanel,
PanelMetadata, PanelTranscription, PanelDiscussion, PanelHistory

PdfViewer: annotateMode and activeAnnotationId lifted to bindable props;
AnnotationCommentPanel removed (discussion moves to the Diskussion tab).

Closes #62
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/PanelDiscussion.svelte

@@ -0,0 +1,85 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+import CommentThread from './CommentThread.svelte';
+
+type CommentReply = {
+	id: string;
+	authorId: string | null;
+	authorName: string;
+	content: string;
+	createdAt: string;
+	updatedAt: string;
+};
+
+type Comment = {
+	id: string;
+	authorId: string | null;
+	authorName: string;
+	content: string;
+	createdAt: string;
+	updatedAt: string;
+	replies: CommentReply[];
+};
+
+type Props = {
+	documentId: string;
+	activeAnnotationId: string | null;
+	initialComments: Comment[];
+	canComment: boolean;
+	currentUserId: string | null;
+	canAdmin: boolean;
+	onAnnotationCommentCountChange?: (annotationId: string, count: number) => void;
+};
+
+let {
+	documentId,
+	activeAnnotationId,
+	initialComments,
+	canComment,
+	currentUserId,
+	canAdmin,
+	onAnnotationCommentCountChange
+}: Props = $props();
+</script>
+
+<div class="space-y-8 p-6">
+	<!-- Annotation thread (shown when an annotation is active) -->
+	{#if activeAnnotationId}
+		<div>
+			<h4
+				class="mb-3 border-b border-brand-sand pb-2 font-sans text-xs font-bold tracking-widest text-brand-navy uppercase"
+			>
+				{m.doc_panel_annotation_thread_title()}
+			</h4>
+			{#key activeAnnotationId}
+				<CommentThread
+					documentId={documentId}
+					annotationId={activeAnnotationId}
+					canComment={canComment}
+					currentUserId={currentUserId}
+					canAdmin={canAdmin}
+					loadOnMount={true}
+					onCountChange={(count) => onAnnotationCommentCountChange?.(activeAnnotationId, count)}
+				/>
+			{/key}
+		</div>
+	{/if}
+
+	<!-- General document discussion -->
+	<div>
+		{#if activeAnnotationId}
+			<h4
+				class="mb-3 border-b border-brand-sand pb-2 font-sans text-xs font-bold tracking-widest text-brand-navy uppercase"
+			>
+				{m.comment_section_title()}
+			</h4>
+		{/if}
+		<CommentThread
+			documentId={documentId}
+			initialComments={initialComments}
+			canComment={canComment}
+			currentUserId={currentUserId}
+			canAdmin={canAdmin}
+		/>
+	</div>
+</div>