feat: add ESLint 9 configuration with TypeScript support

- Add eslint.config.js using new flat config format
- Configure @typescript-eslint/parser and plugin for TypeScript files
- Add separate config for vanilla JavaScript files (gateway/ui)
- Include Node.js and browser globals
- Enable strict rules: curly braces, no-eval, eqeqeq, etc.
- Configure TypeScript-specific rules (no-explicit-any, no-non-null-assertion)
- Add @typescript-eslint/parser and @typescript-eslint/eslint-plugin dependencies

package.json

@@ -32,6 +32,8 @@
     "@types/react": "^19.0.0",
     "@types/turndown": "^5.0.0",
     "@types/ws": "^8.18.1",
+    "@typescript-eslint/eslint-plugin": "^8.55.0",
+    "@typescript-eslint/parser": "^8.55.0",
     "eslint": "^9.0.0",
     "tsx": "^4.0.0",
     "typescript": "^5.7.0",