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

@@ -0,0 +1,16 @@
+import { describe, it, expect } from 'vitest';
+
+// Browser-mode tests must run with SvelteKit's hover-prefetch disabled.
+// Hover-prefetch fires real `fetch` requests for the target route's loader
+// chunks; those go through the same Playwright route handler that serves
+// mocked modules. Even after `cleanup()` tears down the iframe, an in-flight
+// prefetch can still hit the handler — and if the worker's birpc channel has
+// closed by then, the handler raises an unhandled rejection. ADR-012 / #553.
+//
+// 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', () => {
+		expect(document.body.dataset.sveltekitPreloadData).toBe('off');
+	});
+});

frontend/src/test-setup.ts

@@ -0,0 +1,8 @@
+// Disable SvelteKit hover-prefetch in browser-mode tests. ADR-012 / #553.
+//
+// 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
+// route handler with a closed birpc channel, raising an unhandled rejection
+// that exits the run with code 1 even when every individual test was green.
+document.body.dataset.sveltekitPreloadData = 'off';

frontend/vite.config.ts

@@ -77,6 +77,7 @@ 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/**']
 				}

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: {