fix(documents): sync filter display state with URL on navigation

Three root causes prevented filters from reflecting the URL after SvelteKit
client-side navigation:

1. +page.server.ts now resolves sender/receiver display names in parallel with
   the document search (UUID validation + silent 404 drop), so initialSenderName
   / initialReceiverName land in server data ready for the UI to use.

2. +page.svelte passes initialSenderName, initialReceiverName, and navKey
   (incremented via untrack on every navigation) down to SearchFilterBar.
   The untrack() prevents the effect from re-running due to its own navKey write.

3. SearchFilterBar forwards navKey as resetKey to each PersonTypeahead, which
   already had a void resetKey guard added in the previous commit.

Together these ensure that after navigating to /documents?senderId=<uuid> the
typeahead shows the person's display name, and clicking × reset clears it.

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

frontend/src/routes/documents/page.server.spec.ts

@@ -167,3 +167,72 @@ describe('documents page load — network error fallback', () => {
 		expect(result.items).toEqual([]);
 	});
 });
+
+// ─── person name resolution ───────────────────────────────────────────────────
+
+describe('documents page load — person name resolution', () => {
+	function makeSearchMock() {
+		const mockGet = vi.fn().mockResolvedValue({
+			response: { ok: true, status: 200 },
+			data: { items: [], totalElements: 0, pageNumber: 0, pageSize: 50, totalPages: 0 }
+		});
+		vi.mocked(createApiClient).mockReturnValue({ GET: mockGet } as ReturnType<
+			typeof createApiClient
+		>);
+	}
+
+	it('returns initialSenderName from person lookup when senderId is a valid UUID', async () => {
+		makeSearchMock();
+		const mockFetch = vi.fn().mockResolvedValue({
+			ok: true,
+			json: vi.fn().mockResolvedValue({ displayName: 'Max Mustermann' })
+		});
+
+		const result = await load({
+			url: makeUrl({ senderId: '11111111-1111-1111-1111-111111111111' }),
+			fetch: mockFetch as unknown as typeof fetch
+		});
+
+		expect(result.initialSenderName).toBe('Max Mustermann');
+	});
+
+	it('returns initialReceiverName from person lookup when receiverId is a valid UUID', async () => {
+		makeSearchMock();
+		const mockFetch = vi.fn().mockResolvedValue({
+			ok: true,
+			json: vi.fn().mockResolvedValue({ displayName: 'Anna Musterfrau' })
+		});
+
+		const result = await load({
+			url: makeUrl({ receiverId: '22222222-2222-2222-2222-222222222222' }),
+			fetch: mockFetch as unknown as typeof fetch
+		});
+
+		expect(result.initialReceiverName).toBe('Anna Musterfrau');
+	});
+
+	it('returns empty string when senderId is not a valid UUID', async () => {
+		makeSearchMock();
+		const mockFetch = vi.fn();
+
+		const result = await load({
+			url: makeUrl({ senderId: 'not-a-uuid' }),
+			fetch: mockFetch as unknown as typeof fetch
+		});
+
+		expect(result.initialSenderName).toBe('');
+		expect(mockFetch).not.toHaveBeenCalledWith(expect.stringContaining('/api/persons/'));
+	});
+
+	it('returns empty string when person fetch returns 404', async () => {
+		makeSearchMock();
+		const mockFetch = vi.fn().mockResolvedValue({ ok: false, status: 404 });
+
+		const result = await load({
+			url: makeUrl({ senderId: '11111111-1111-1111-1111-111111111111' }),
+			fetch: mockFetch as unknown as typeof fetch
+		});
+
+		expect(result.initialSenderName).toBe('');
+	});
+});