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/test-setup.ts

@@ -1,8 +1,14 @@
-// Disable SvelteKit hover-prefetch in browser-mode tests. ADR-012 / #553.
+// Disable SvelteKit hover-prefetch (both data + code) 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.
+//
+// `data-sveltekit-preload-data="off"` disables loader-data prefetch;
+// `data-sveltekit-preload-code="off"` disables route-code chunk prefetch.
+// Both surfaces can produce late module fetches that hit the route handler.
 document.body.dataset.sveltekitPreloadData = 'off';
+document.body.dataset.sveltekitPreloadCode = 'off';