feat(test): add Istanbul browser coverage via standalone client config

Vitest 4 silently ignores per-project coverage overrides in test.projects,
so a standalone vitest.client-coverage.config.ts provides the root-level
Istanbul coverage block that Vitest actually honours.

Root vite.config.ts retains the v8 coverage block (reportsDirectory:
coverage/server) for the server project. The client config writes to
coverage/client and instruments all .svelte and .svelte.ts files.

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

frontend/vite.config.ts

@@ -46,6 +46,7 @@ export default defineConfig({
 		coverage: {
 			provider: 'v8',
 			reporter: ['text', 'lcov'],
+			reportsDirectory: 'coverage/server',
 			// Measure utility and server-side logic only. Svelte components run
 			// in the browser project and are excluded here. Browser-only TS files
 			// (actions, hooks, domain-specific UI state) are also excluded.

frontend/vitest.client-coverage.config.ts

@@ -0,0 +1,46 @@
+import { paraglideVitePlugin } from '@inlang/paraglide-js';
+import devtoolsJson from 'vite-plugin-devtools-json';
+import tailwindcss from '@tailwindcss/vite';
+import { defineConfig } from 'vitest/config';
+import { playwright } from '@vitest/browser-playwright';
+import { sveltekit } from '@sveltejs/kit/vite';
+
+// Standalone config for browser-project Istanbul coverage.
+// Uses a dedicated root-level coverage block because Vitest 4 ignores
+// per-project coverage overrides inside test.projects.
+export default defineConfig({
+	optimizeDeps: {
+		include: ['pdfjs-dist', '@tiptap/core', '@tiptap/starter-kit', '@tiptap/extension-mention']
+	},
+	plugins: [
+		tailwindcss(),
+		sveltekit(),
+		devtoolsJson(),
+		paraglideVitePlugin({
+			project: './project.inlang',
+			outdir: './src/lib/paraglide'
+		})
+	],
+	test: {
+		expect: { requireAssertions: true },
+		browser: {
+			enabled: true,
+			provider: playwright(),
+			instances: [{ browser: 'chromium', headless: true }],
+			screenshotDirectory: 'test-results/screenshots',
+			screenshotFailures: true
+		},
+		include: ['src/**/*.svelte.{test,spec}.{js,ts}'],
+		exclude: ['src/lib/server/**'],
+		coverage: {
+			provider: 'istanbul',
+			reporter: ['text', 'lcov'],
+			reportsDirectory: 'coverage/client',
+			include: ['src/**/*.svelte', 'src/**/*.svelte.ts'],
+			exclude: ['src/lib/paraglide/**', 'src/lib/generated/**', 'src/hooks/**', '**/__mocks__/**'],
+			thresholds: {
+				branches: 80
+			}
+		}
+	}
+});