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>

.prettierignore

@@ -0,0 +1,6 @@
+build/
+.svelte-kit/
+node_modules/
+package-lock.json
+static/
+*.md

.prettierrc

@@ -0,0 +1,15 @@
+{
+	"useTabs": true,
+	"singleQuote": true,
+	"trailingComma": "none",
+	"printWidth": 100,
+	"plugins": ["prettier-plugin-svelte"],
+	"overrides": [
+		{
+			"files": "*.svelte",
+			"options": {
+				"parser": "svelte"
+			}
+		}
+	]
+}

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'
+			}
+		}
+	}
+];

package.json

@@ -9,17 +9,30 @@
 		"preview": "vite preview",
 		"prepare": "svelte-kit sync || echo ''",
 		"check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
-		"check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch"
+		"check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch",
+		"lint": "prettier --check . && eslint .",
+		"format": "prettier --write .",
+		"format:check": "prettier --check .",
+		"test": "vitest run",
+		"test:watch": "vitest"
 	},
 	"devDependencies": {
 		"@sveltejs/adapter-auto": "^7.0.0",
 		"@sveltejs/adapter-static": "^3.0.10",
+		"@sveltejs/eslint-config": "^10.0.1",
 		"@sveltejs/kit": "^2.49.1",
 		"@sveltejs/vite-plugin-svelte": "^6.2.1",
+		"eslint": "^10.4.1",
+		"eslint-plugin-svelte": "^3.19.0",
+		"globals": "^17.6.0",
+		"prettier": "^3.8.3",
+		"prettier-plugin-svelte": "^4.1.0",
 		"svelte": "^5.45.6",
 		"svelte-check": "^4.3.4",
 		"typescript": "^5.9.3",
-		"vite": "^7.2.6"
+		"typescript-eslint": "^8.60.1",
+		"vite": "^7.2.6",
+		"vitest": "^4.1.8"
 	},
 	"dependencies": {
 		"gsap": "^3.14.2",

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']
+	}
 });