chore: add ESLint tooling with TypeScript support

The repository has a pnpm lint script running 'eslint .' but ESLint
and typescript-eslint were not listed in devDependencies.

Added:
- eslint (^9.0.0)
- @eslint/js (^9.0.0)
- typescript-eslint (^8.0.0)

Added eslint.config.mjs with flat config (ESLint 9+) using
typescript-eslint strict rules. Ignores node_modules, dist, build,
and framework-specific output directories.

package.json

@@ -12,7 +12,10 @@
     "lint": "eslint ."
   },
   "devDependencies": {
+    "@eslint/js": "^9.0.0",
+    "eslint": "^9.0.0",
     "typescript": "^5.7.0",
+    "typescript-eslint": "^8.0.0",
     "vitest": "^3.1.0"
   },
   "pnpm": {