test(setup): disable SvelteKit hover prefetch in browser-mode runs

Hover-prefetch fires real fetch requests for route loader chunks; those
requests go through the same Playwright route handler that serves mocked
modules. An in-flight prefetch landing after iframe teardown can hit the
handler with a closed birpc channel, raising an unhandled rejection that
exits the run with code 1 even when every individual test was green.

Add `src/test-setup.ts` that sets `document.body.dataset.sveltekitPreloadData
= 'off'` and wire it via `setupFiles` in both `vite.config.ts` (client
project) and `vitest.client-coverage.config.ts` (Istanbul coverage config).
Add `src/__meta__/browser-preload-disabled.svelte.test.ts` asserting the
setup ran. Zero production impact.

Issue #553 secondary trigger.

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

frontend/vitest.client-coverage.config.ts

@@ -32,6 +32,7 @@ export default defineConfig({
 			screenshotDirectory: 'test-results/screenshots',
 			screenshotFailures: true
 		},
+		setupFiles: ['./src/test-setup.ts'],
 		include: ['src/**/*.svelte.{test,spec}.{js,ts}'],
 		exclude: ['src/lib/server/**'],
 		coverage: {