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/e2e/appointments.spec.ts

@@ -1,5 +1,5 @@
 import { test, expect, Page } from "@playwright/test";
-import { formatDateLocal, getTomorrowDateStr } from "./helpers/date";
+import { getTomorrowDateStr } from "./helpers/date";
 import { API_URL, REGULAR_USER, ADMIN_USER, clearAuth, loginUser } from "./helpers/auth";
 
 /**