feat(bulk-edit): extend BulkDocumentEditLayout with mode="edit"

- New FieldLabelBadge component (additive / replace variants, WCAG AA contrast)
- WhoWhenSection: hideDate prop, editMode prop renders badges next to sender
  and receivers, hides the meta_location field
- DescriptionSection: editMode prop renders badges next to tags and archive
  fields; new bindable archiveBox / archiveFolder inputs only in editMode
- PersonTypeahead: optional badge prop forwards to FieldLabelBadge
- FileSwitcherStrip FileEntry: file is now optional, documentId added so
  edit-mode entries reference an existing document by UUID
- BulkDocumentEditLayout: mode prop branches drop zone / read-only title /
  callout / save handler. Edit save chunks 500 IDs per PATCH, stops on chunk
  failure with retry, marks per-document errors as chips, clears the bulk
  selection store on full success.

Refs #225

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

frontend/src/lib/components/document/FieldLabelBadge.svelte.spec.ts

@@ -0,0 +1,30 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import FieldLabelBadge from './FieldLabelBadge.svelte';
+
+afterEach(() => cleanup());
+
+describe('FieldLabelBadge', () => {
+	it('renders the additive variant text', async () => {
+		render(FieldLabelBadge, { variant: 'additive' });
+		await expect.element(page.getByTestId('field-label-badge-additive')).toBeInTheDocument();
+		await expect
+			.element(page.getByTestId('field-label-badge-additive'))
+			.toHaveTextContent('+ wird hinzugefügt');
+	});
+
+	it('renders the replace variant text', async () => {
+		render(FieldLabelBadge, { variant: 'replace' });
+		await expect
+			.element(page.getByTestId('field-label-badge-replace'))
+			.toHaveTextContent('wird ersetzt');
+	});
+
+	it('uses text-gray-600 for WCAG-AA contrast on muted backgrounds', async () => {
+		render(FieldLabelBadge, { variant: 'replace' });
+		await expect
+			.element(page.getByTestId('field-label-badge-replace'))
+			.toHaveClass(/text-gray-600/);
+	});
+});