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/FileSwitcherStrip.svelte.spec.ts

@@ -10,6 +10,7 @@ export interface FileEntry {
 	file: File;
 	title: string;
 	status: 'idle' | 'error';
+	previewUrl: string;
 }
 
 function makeFiles(n: number): FileEntry[] {
@@ -17,7 +18,8 @@ function makeFiles(n: number): FileEntry[] {
 		id: `id-${i}`,
 		file: new File([''], `file${i}.pdf`),
 		title: `File ${i}`,
-		status: 'idle' as const
+		status: 'idle' as const,
+		previewUrl: ''
 	}));
 }
 
@@ -65,7 +67,13 @@ describe('FileSwitcherStrip', () => {
 
 	it('error chip has aria-label containing warning indicator', async () => {
 		const files: FileEntry[] = [
-			{ id: 'e1', file: new File([''], 'bad.pdf'), title: 'Bad file', status: 'error' }
+			{
+				id: 'e1',
+				file: new File([''], 'bad.pdf'),
+				title: 'Bad file',
+				status: 'error',
+				previewUrl: ''
+			}
 		];
 		const { container } = render(FileSwitcherStrip, {
 			files,
@@ -85,7 +93,7 @@ describe('FileSwitcherStrip', () => {
 			onSelect: vi.fn(),
 			onRemove: vi.fn()
 		});
-		const firstBtn = container.querySelectorAll('[role="button"]')[0] as HTMLElement;
+		const firstBtn = container.querySelectorAll('[data-chip-id]')[0] as HTMLElement;
 		firstBtn.focus();
 		await userEvent.keyboard('{ArrowRight}');
 		const focused = document.activeElement;