fix(bulk-upload): spec-compliant split-panel layout with local PDF preview

Rewrites BulkDocumentEditLayout to match the spec exactly:
- Fixed viewport layout (same as DocumentEditLayout) filling viewport below nav
- Split panel visible in all states (N=0/1/≥2) — was fullscreen dark drop zone
- N=0: centered drop-zone-box in left panel; shared form visible but greyed out
- N≥1: real PDF preview via URL.createObjectURL (no server upload required)
- N≥2: FileSwitcherStrip at bottom of left panel; count pill + discard in topbar
- FileEntry gains previewUrl; blob URLs created on add, revoked on remove/destroy
- save() checks response.ok and marks failed files with status: 'error'
- BulkDropZone redesigned: spec-accurate box with circular mint icon, serif title
- FileSwitcherStrip: number badges, arrows, keyboard nav via data-chip-id selector
- ScopeCard, UploadSaveBar: hardcoded German replaced with Paraglide i18n keys
- +page.svelte simplified to bare component render (layout is self-contained)

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

frontend/src/routes/documents/new/+page.svelte

@@ -1,37 +1,11 @@
 <script lang="ts">
-import { m } from '$lib/paraglide/messages.js';
 import BulkDocumentEditLayout from '$lib/components/document/BulkDocumentEditLayout.svelte';
 
 let { data } = $props();
 </script>
 
-<div class="mx-auto max-w-7xl px-4 py-8">
-	<div class="mb-6">
-		<a
-			href="/"
-			class="group mb-4 inline-flex items-center text-xs font-bold tracking-widest text-ink-2 uppercase transition-colors hover:text-ink"
-		>
-			<svg
-				class="mr-2 h-4 w-4 transform transition-transform group-hover:-translate-x-1"
-				fill="none"
-				stroke="currentColor"
-				viewBox="0 0 24 24"
-			>
-				<path
-					stroke-linecap="round"
-					stroke-linejoin="round"
-					stroke-width="2"
-					d="M10 19l-7-7m0 0l7-7m-7 7h18"
-				/>
-			</svg>
-			{m.btn_back_to_overview()}
-		</a>
-		<h1 class="font-serif text-3xl text-ink">{m.doc_new_heading()}</h1>
-	</div>
-
-	<BulkDocumentEditLayout
-		initialSenderId={data.initialSenderId}
-		initialSenderName={data.initialSenderName}
-		initialReceivers={data.initialReceivers}
-	/>
-</div>
+<BulkDocumentEditLayout
+	initialSenderId={data.initialSenderId}
+	initialSenderName={data.initialSenderName}
+	initialReceivers={data.initialReceivers}
+/>