test(#125): remove demo.spec.ts scaffold leftover

Deletes the npm create svelte scaffold file that tested arithmetic
instead of application code. Inflated the test count and added noise
to coverage reports.

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

frontend/src/demo.spec.ts

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