fix(bulk-edit): cycle-3 polish — Felix C2/C3/C4/C5 + Sara coverage gaps

Felix C2 — `BatchMetadataRequest` controller now uses `@Valid` so future
@Size/etc. annotations on the record actually fire.

Felix C3 — Auto-clear `$effect` in `+layout.svelte` reads
`bulkSelectionStore.size` inside `untrack()` so the effect only re-fires on
route change, not on every checkbox toggle.

Felix C4 — `BulkDocumentEditLayout` edit-mode hydration loop now lives
inside `onMount` (not at top-level script) so the SvelteMap mutation is
unambiguously tied to instance lifecycle, matching the pattern used by
`WhoWhenSection`/`DescriptionSection` after the cycle-2 fix.

Felix C5 — Replaced fully-qualified `java.util.LinkedHashSet` in
`DocumentController` with a top-of-file import.

Sara coverage — six new spec files / blocks pin the cycle-1 and cycle-2
behaviours that were previously untested:
 - `WhoWhenSection.svelte.spec.ts` — onMount seeding from initialDateIso /
   initialLocation; doesn't stomp parent-bound dateIso; hideDate / editMode
   branch
 - `DescriptionSection.svelte.spec.ts` — onMount seeding from initialTitle /
   initialDocumentLocation; doesn't stomp parent-bound values; archive-box /
   archive-folder fields visible only in editMode
 - `BulkSelectionBar.svelte.spec.ts` — Esc-scope guard tests for `<dialog>`
   open and `aria-expanded` popover present
 - `BulkDocumentEditLayout.svelte.spec.ts` — topbar reads
   "Massenbearbeitung" + "werden bearbeitet" in edit mode (not the
   upload-flavoured "hochladen"/"werden erstellt" copy)
 - `DocumentControllerTest.patchBulk_returns400_whenArchiveBoxExceeds255Chars`
   — pins the @Size validator on archiveBox via the @Valid wiring

Refs #225, PR #331

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

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

@@ -0,0 +1,42 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import WhoWhenSection from './WhoWhenSection.svelte';
+
+afterEach(() => cleanup());
+
+describe('WhoWhenSection — onMount seeding (Felix B1 fix regression fence)', () => {
+	it('pre-fills the date input from initialDateIso when the bindable is empty', async () => {
+		render(WhoWhenSection, { initialDateIso: '2024-03-15' });
+		// isoToGerman('2024-03-15') → '15.03.2024'
+		const dateInput = document.querySelector('input#documentDate') as HTMLInputElement;
+		expect(dateInput).not.toBeNull();
+		expect(dateInput.value).toBe('15.03.2024');
+	});
+
+	it('does not stomp a parent-bound dateIso that is already non-empty', async () => {
+		// dateIso bindable is '2026-01-01' from the parent; initialDateIso is the
+		// "fallback seed". onMount must not overwrite the already-bound value.
+		render(WhoWhenSection, { dateIso: '2026-01-01', initialDateIso: '1900-01-01' });
+		const dateInput = document.querySelector('input#documentDate') as HTMLInputElement;
+		expect(dateInput.value).toBe('01.01.2026');
+	});
+
+	it('hides the date field when hideDate=true (bulk-edit mode)', async () => {
+		render(WhoWhenSection, { hideDate: true });
+		await expect.element(page.getByTestId('who-when-date')).not.toBeInTheDocument();
+	});
+
+	it('renders the meta_location input only outside editMode', async () => {
+		const { rerender } = render(WhoWhenSection, { editMode: true });
+		expect(document.querySelector('input#location')).toBeNull();
+		await rerender({ editMode: false });
+		expect(document.querySelector('input#location')).not.toBeNull();
+	});
+
+	it('pre-fills the location input from initialLocation', async () => {
+		render(WhoWhenSection, { editMode: false, initialLocation: 'Berlin' });
+		const locationInput = document.querySelector('input#location') as HTMLInputElement;
+		expect(locationInput.value).toBe('Berlin');
+	});
+});