chore: add ESLint, Prettier, and Vitest tooling

Add code-quality tooling the project lacked:
- ESLint (flat config via @sveltejs/eslint-config)
- Prettier (tabs, single quotes, matching existing style)
- Vitest (node env, for pure-logic unit tests)
- npm scripts: lint, format, format:check, test, test:watch

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

vite.config.ts

@@ -1,6 +1,12 @@
+/// <reference types="vitest/config" />
 import { sveltekit } from '@sveltejs/kit/vite';
 import { defineConfig } from 'vite';
 
 export default defineConfig({
-	plugins: [sveltekit()]
+	plugins: [sveltekit()],
+	test: {
+		// Pure-logic unit tests (utils, data). No DOM needed.
+		environment: 'node',
+		include: ['src/**/*.{test,spec}.ts']
+	}
 });