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/PanelTranscription.svelte

@@ -0,0 +1,42 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+
+type Doc = {
+	summary?: string | null;
+	transcription?: string | null;
+};
+
+let { doc }: { doc: Doc } = $props();
+</script>
+
+<div class="flex justify-center px-6 py-8">
+	<div class="w-full max-w-prose space-y-8">
+		{#if !doc.summary && !doc.transcription}
+			<p class="font-serif text-sm text-gray-400 italic">—</p>
+		{/if}
+
+		{#if doc.summary}
+			<div>
+				<span
+					class="mb-3 block font-sans text-xs font-bold tracking-widest text-gray-400 uppercase"
+				>
+					{m.doc_label_summary()}
+				</span>
+				<p class="font-serif text-base leading-relaxed text-brand-navy">{doc.summary}</p>
+			</div>
+		{/if}
+
+		{#if doc.transcription}
+			<div>
+				<span
+					class="mb-3 block font-sans text-xs font-bold tracking-widest text-gray-400 uppercase"
+				>
+					{m.form_label_transcription()}
+				</span>
+				<p class="font-serif text-base leading-relaxed whitespace-pre-wrap text-brand-navy">
+					{doc.transcription}
+				</p>
+			</div>
+		{/if}
+	</div>
+</div>