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>

eslint.config.js

@@ -0,0 +1,20 @@
+import svelteConfig from '@sveltejs/eslint-config';
+
+/** @type {import('eslint').Linter.Config[]} */
+export default [
+	{
+		ignores: ['build/', '.svelte-kit/', 'static/', 'node_modules/']
+	},
+	...svelteConfig,
+	{
+		// Service worker runs in a ServiceWorkerGlobalScope, not the browser/node defaults.
+		files: ['static/service-worker.js'],
+		languageOptions: {
+			globals: {
+				self: 'readonly',
+				caches: 'readonly',
+				fetch: 'readonly'
+			}
+		}
+	}
+];