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/BulkSelectionBar.svelte.spec.ts

@@ -86,4 +86,37 @@ describe('BulkSelectionBar', () => {
 		// Nothing to clear, no error.
 		expect(bulkSelectionStore.size).toBe(0);
 	});
+
+	it('Escape does not clear when an open <dialog> is present (Leonie B6 scope guard)', async () => {
+		bulkSelectionStore.add('a');
+		bulkSelectionStore.add('b');
+		render(BulkSelectionBar, { canWrite: true });
+
+		// Simulate a ConfirmDialog being open in front of the bar.
+		const overlay = document.createElement('dialog');
+		overlay.setAttribute('open', '');
+		document.body.appendChild(overlay);
+		try {
+			window.dispatchEvent(new KeyboardEvent('keydown', { key: 'Escape' }));
+			// Escape is captured by the dialog, not the bar — selection survives.
+			expect(bulkSelectionStore.size).toBe(2);
+		} finally {
+			overlay.remove();
+		}
+	});
+
+	it('Escape does not clear when an aria-expanded popover is present', async () => {
+		bulkSelectionStore.add('a');
+		render(BulkSelectionBar, { canWrite: true });
+
+		const trigger = document.createElement('button');
+		trigger.setAttribute('aria-expanded', 'true');
+		document.body.appendChild(trigger);
+		try {
+			window.dispatchEvent(new KeyboardEvent('keydown', { key: 'Escape' }));
+			expect(bulkSelectionStore.size).toBe(1);
+		} finally {
+			trigger.remove();
+		}
+	});
 });