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.