Update frontend to next 16

[stianjsu]

Update the Next.js frontend from v13 to v16, bump Node.js from 20 to 24, and modernize the surrounding toolchain (ESLint 9 flat config, stricter TypeScript, latest dependency versions). Also fixes cookie forwarding for the login flow and corrects optimistic-update bugs in the API hooks.

Loooots of things were deprecated and had tons of sec vuln. Heroku would not deploy

Key additions

• Next.js 13 → 16 — all next/image usages updated with explicit style={{ maxWidth: '100%', height: 'auto' }} to match the new image optimization defaults
• Node.js 20 → 24 — Docker dev image and engines field updated
• ESLint flat config — migrated from .eslintrc.json to eslint.config.mjs with ESLint 9; dropped @typescript-eslint/eslint-plugin in favor of eslint-config-next/typescript; disabled new React Compiler lint rules (react-hooks/purity, react-hooks/set-state-in-effect, react-hooks/refs) that don't apply yet
• TypeScript — jsx changed from "preserve" to "react-jsx" for Next 16 compatibility; formatting normalized to 2-space indent
• CSRF / cookie fix — login callback now uses getSetCookie() to forward multiple Set-Cookie headers (auth + CSRF token) instead of collapsing them with get('set-cookie')
• API hook fixes — useEvents.createEvent now types the response as ApiEvent instead of ApiEventPost; useRestaurants.createRestaurant uses the server response for optimistic cache update instead of the local input object
• Dependency bumps — TypeScript 5.1 → 5.9, Tailwind 3.3 → 3.4, Prettier 3.0 → 3.8, autoprefixer/postcss pinned → caret ranges, and ~40 transitive dependency updates

How Has This Been Tested?

• npm run build — production build passes with the new Next.js version
• npm run lint — ESLint 9 flat config lints cleanly with zero warnings
• npm run typecheck — TypeScript strict mode passes with the updated tsconfig.json