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>

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",