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/ScopeCard.svelte

@@ -1,4 +1,6 @@
 <script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+
 let {
 	variant,
 	count = 0,
@@ -13,24 +15,26 @@ let {
 <div
 	data-testid="scope-card"
 	data-variant={variant}
-	class="rounded-sm border p-4
+	class="mb-3 rounded-sm border p-4
 		{variant === 'per-file'
-		? 'border-brand-mint bg-brand-mint/10'
-		: 'border-brand-sand bg-white'}"
+		? 'border-accent bg-accent-bg'
+		: 'border-line bg-surface'}"
 >
 	{#if variant === 'shared'}
 		<div class="mb-3 flex items-center justify-between">
-			<span class="text-xs font-bold tracking-widest text-gray-400 uppercase">
-				Gilt für alle Dateien
+			<span class="text-xs font-bold tracking-widest text-ink-3 uppercase">
+				{m.bulk_scope_shared_label({ count })}
 			</span>
 			<span
-				class="bg-brand-sand inline-flex h-5 min-w-5 items-center justify-center rounded-full px-1.5 text-xs font-bold text-brand-navy"
+				class="inline-flex h-5 min-w-5 items-center justify-center rounded-full bg-accent px-1.5 text-xs font-bold text-primary"
 			>
 				{count}
 			</span>
 		</div>
 	{:else}
-		<p class="mb-3 text-xs font-bold tracking-widest text-brand-mint uppercase">Diese Datei</p>
+		<p class="mb-3 text-xs font-bold tracking-widest text-accent uppercase">
+			{m.bulk_scope_per_file_label()}
+		</p>
 	{/if}
 	{@render children?.()}
 </div>