fix: address ESLint warnings and add Vitest configuration

- Add ESLint flat config with Svelte and TypeScript support
- Fix unused variables and component prop naming
- Add Vitest browser testing setup with Playwright
- Configure separate test projects for client and server code

web/src/demo.spec.ts

@@ -0,0 +1,7 @@
+import { describe, it, expect } from 'vitest';
+
+describe('sum test', () => {
+	it('adds 1 + 2 to equal 3', () => {
+		expect(1 + 2).toBe(3);
+	});
+});