test(e2e): block the service worker except where the test is about it

The caching worker controls every page once it registers, and Playwright's
page.route() does not see the fetches a worker makes — so the failed-load test,
which forces a 500 to check the error path, got the real 200 through the worker
and read as a broken feature. Block the worker by default and turn it back on only
for the offline test, which is the one that needs a real one.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/playwright.config.js

@@ -51,6 +51,12 @@ export default defineConfig({
 
 	use: {
 		baseURL,
+		// Off by default. The app now ships a caching service worker, and once it
+		// controls a page Playwright's page.route() no longer sees the fetches it
+		// makes — so a test that forces a 500 to check the error path gets the real
+		// 200 through the worker instead, and reads as a broken feature. The one test
+		// that needs the real worker turns it back on for its own context.
+		serviceWorkers: 'block',
 		trace: 'retain-on-failure',
 		screenshot: 'only-on-failure',
 		// Pinned, because the app reads both. The sign-up form sends