Add ESLint for TypeScript/React linting

- Install eslint, typescript-eslint, eslint-plugin-react-hooks
- Configure eslint.config.js with flat config format
- Add type: module to package.json
- Fix unused variable issues (prefix with underscore)
- Add Makefile targets: lint-frontend, fix-frontend

frontend/package.json

@@ -2,13 +2,16 @@
   "name": "frontend",
   "version": "0.1.0",
   "private": true,
+  "type": "module",
   "scripts": {
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
     "test": "vitest run",
     "test:e2e": "playwright test",
-    "generate-api-types": "openapi-typescript http://localhost:8000/openapi.json -o app/generated/api.ts"
+    "generate-api-types": "openapi-typescript http://localhost:8000/openapi.json -o app/generated/api.ts",
+    "lint": "eslint .",
+    "lint:fix": "eslint . --fix"
   },
   "dependencies": {
     "bech32": "^2.0.0",
@@ -17,14 +20,18 @@
     "react-dom": "19.0.0"
   },
   "devDependencies": {
+    "@eslint/js": "^9.39.2",
     "@playwright/test": "^1.49.1",
     "@testing-library/react": "^16.1.0",
     "@types/node": "25.0.3",
     "@types/react": "19.2.7",
     "@vitejs/plugin-react": "^4.3.4",
+    "eslint": "^9.39.2",
+    "eslint-plugin-react-hooks": "^7.0.1",
     "jsdom": "^26.0.0",
     "openapi-typescript": "^7.10.1",
     "typescript": "5.9.3",
+    "typescript-eslint": "^8.50.0",
     "vitest": "^2.1.8"
   }
 }