Vitest: client and server test environments

One app, two test environments: Svelte component tests need a real browser, server code needs node. Same split issues uses inside its web app.

• client project: browser env via the Playwright provider, *.svelte.test.ts, excludes src/lib/server
• server project: node env, *.test.ts; DB-touching tests get fileParallelism false + NODE_ENV=test
• expect.requireAssertions true in both; one seed test each proves the harness