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/lib/components/document/BulkDropZone.svelte

@@ -1,4 +1,6 @@
 <script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+
 let {
 	onFilesAdded
 }: {
@@ -8,30 +10,35 @@ let {
 let isDragging = $state(false);
 </script>
 
-<div class="flex min-h-[300px] flex-1 flex-col items-center justify-center bg-pdf-bg">
+<div
+	role="region"
+	aria-label="Dateien ablegen"
+	data-testid="bulk-drop-zone"
+	class="flex flex-1 flex-col items-center justify-center p-6"
+	ondragover={(e) => {
+		e.preventDefault();
+		isDragging = true;
+	}}
+	ondragleave={() => (isDragging = false)}
+	ondrop={(e) => {
+		e.preventDefault();
+		isDragging = false;
+		if (e.dataTransfer && e.dataTransfer.files.length > 0) {
+			onFilesAdded(Array.from(e.dataTransfer.files));
+		}
+	}}
+>
 	<div
-		role="region"
-		aria-label="Dateien ablegen"
-		data-testid="bulk-drop-zone"
-		class="flex flex-col items-center gap-4 rounded-sm border border-dashed p-12 text-center transition-colors
-			{isDragging ? 'border-brand-mint ring-2 ring-brand-mint' : 'border-white/20'}"
-		ondragover={(e) => {
-			e.preventDefault();
-			isDragging = true;
-		}}
-		ondragleave={() => (isDragging = false)}
-		ondrop={(e) => {
-			e.preventDefault();
-			isDragging = false;
-			if (e.dataTransfer && e.dataTransfer.files.length > 0) {
-				onFilesAdded(Array.from(e.dataTransfer.files));
-			}
-		}}
+		class={[
+			'flex w-full max-w-sm flex-col items-center gap-3 rounded-md border-2 border-dashed px-6 py-9 text-center transition-colors',
+			isDragging ? 'border-accent bg-accent/10' : 'border-accent/50 bg-white/[0.04]'
+		].join(' ')}
 	>
-		<div class="flex h-8 w-8 items-center justify-center rounded-full bg-white/10 text-white/40">
+		<!-- Circular mint icon -->
+		<div class="flex h-14 w-14 items-center justify-center rounded-full bg-accent text-primary">
 			<svg
-				width="16"
-				height="16"
+				width="22"
+				height="22"
 				viewBox="0 0 32 32"
 				xmlns="http://www.w3.org/2000/svg"
 				aria-hidden="true"
@@ -42,11 +49,20 @@ let isDragging = $state(false);
 				/>
 			</svg>
 		</div>
-		<p class="text-sm font-medium text-white/60">PDF-Dateien hier ablegen</p>
-		<p class="text-xs text-white/30">oder</p>
+
+		<!-- Serif title -->
+		<p class="font-serif text-sm font-bold text-ink">{m.bulk_drop_hint()}</p>
+
+		<!-- Sub description -->
+		<p class="text-xs leading-relaxed text-ink-2">
+			Für jede Datei wird ein eigenes Dokument erstellt.<br />
+			<strong class="text-ink">Der Titel</strong> wird aus dem Dateinamen vorausgefüllt —
+			<strong class="text-ink">alle anderen Felder</strong> gelten für alle gemeinsam.
+		</p>
+
+		<!-- CTA button -->
 		<label
-			class="flex min-h-[44px] cursor-pointer items-center rounded-sm bg-brand-navy px-4 py-1.5 text-xs font-bold tracking-widest text-white/90 uppercase"
-			aria-label="Dateien auswählen"
+			class="flex min-h-[44px] cursor-pointer items-center rounded-sm bg-primary px-4 py-2 text-xs font-bold tracking-widest text-primary-fg uppercase transition-opacity hover:opacity-90"
 		>
 			Dateien auswählen
 			<input
@@ -60,5 +76,8 @@ let isDragging = $state(false);
 				}}
 			/>
 		</label>
+
+		<!-- Format hint -->
+		<p class="text-[11px] text-ink-3">{m.bulk_drop_sub()}</p>
 	</div>
 </div>