test(setup): also disable data-sveltekit-preload-code in browser tests

Hover-prefetch has two surfaces in SvelteKit:
- data-sveltekit-preload-data (route loader data)
- data-sveltekit-preload-code (route JS chunks)

The original fix turned off only the loader-data side. Route-code chunks
prefetched on hover can also include manually-mocked module URLs; an
in-flight code prefetch landing after iframe teardown hits the same
Playwright route handler that resolves manual mocks, raising the
unhandled rejection. Disable both surfaces.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/__meta__/browser-preload-disabled.svelte.test.ts

@@ -10,7 +10,11 @@ import { describe, it, expect } from 'vitest';
 // This test enforces that the test-setup file ran and switched preload-data
 // off on `document.body` before any spec started rendering.
 describe('browser test setup', () => {
-	it('disables SvelteKit hover prefetch on document.body', () => {
+	it('disables SvelteKit loader-data prefetch on document.body', () => {
 		expect(document.body.dataset.sveltekitPreloadData).toBe('off');
 	});
+
+	it('disables SvelteKit route-code prefetch on document.body', () => {
+		expect(document.body.dataset.sveltekitPreloadCode).toBe('off');
+	});
 });