feat: add code coverage with artifact upload for unit and E2E tests

Add V8 coverage for unit tests via @vitest/coverage-v8 and Istanbul
instrumentation for E2E tests via vite-plugin-istanbul. Coverage data
is collected from browser pages after each Playwright test and merged
with nyc. CI uploads both coverage reports as GitHub Actions artifacts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/e2e/helpers/fixtures.ts

@@ -1,4 +1,5 @@
 import { test as base, expect, type Page, type Locator } from '@playwright/test';
+import { collectCoverage } from './coverage';
 
 const TEST_EMAIL = 'admin@test.com';
 const TEST_PASSWORD = 'testpassword123';
@@ -13,10 +14,12 @@ export function noteCard(page: Page, title: string): Locator {
  * Extended test fixture that handles setup/auth.
  */
 export const test = base.extend<{ authenticatedPage: Page }>({
-	authenticatedPage: async ({ page }, use) => {
+	authenticatedPage: async ({ page }, use, testInfo) => {
 		// Setup: create user and login
 		await setupAndLogin(page);
 		await use(page);
+		// Teardown: collect coverage data from the browser
+		await collectCoverage(page, testInfo.title);
 	}
 });