[IanHollow]

Description

Upgrade the frontend from Next.js 14 to 16.3.8. The App Router upgrade also moves React and React DOM to 19.3.0. Update NextAuth to its current v4 release and Lucide to a version that supports React 19.

The upgrade replaces the removed next lint command with the ESLint CLI and a flat config. It moves the Next config to TypeScript, removes options that Next 16 no longer accepts, and lets the production build check TypeScript. The existing standalone output and runtime API proxy remain in place. Contributor guidance now states Next's Node.js minimum.

Type of change

• CI/CD or build changes
• Refactoring (no intended user-facing change)

Checklist

• Read CONTRIBUTING.md and the Next.js 15 and 16 upgrade guides.
• Updated the lockfile without peer dependency overrides.
• Updated the TypeScript and lint configuration for Next 16.
• Ran the frontend checks and a standalone server smoke test.

Testing

• npm ci completed without peer dependency overrides.
• npm run typecheck, npm run lint, npm run i18n:check, and npm run vocab:check passed.
• npm test -- --run passed: 19 test files, 194 tests.
• npm run build passed with Turbopack and TypeScript validation.
• The standalone server returned HTTP 200 for /, /login, and /api/auth/providers.
• git diff --check passed.

Docker was unavailable locally, so I tested the standalone output directly. The new ESLint preset reports 79 warnings in existing code. Five newly enforced rules are set to warning severity so this framework upgrade does not require unrelated UI changes; lint still fails on all other errors.

Additional notes

PRs #237 and #246 also change the frontend package files. Whichever merges second will need a rebase and another install/build check.

Summary by Sourcery

Upgrade the frontend toolchain to Next.js 16 and React 19 while modernizing its build, lint, and type-check configuration.

Enhancements:

• Upgrade the frontend to Next.js 16 with React 19 and compatible NextAuth and Lucide dependencies.
• Adopt Next.js 16-compatible TypeScript, ESLint flat configuration, and typed Next configuration while preserving standalone builds and runtime API proxying.

Build:

• Replace the removed Next.js lint command with the ESLint CLI and add a dedicated TypeScript typecheck script.

Documentation:

• Document the updated Node.js requirement and frontend typecheck command.

Tests:

• Update affected TypeScript code and tests for the upgraded toolchain.

Chores:

• Refresh frontend dependencies and lockfile for the framework upgrade.

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Reviewer's Guide

Upgrades the frontend from Next.js 14/React 18 to Next.js 16.3.8/React 19.3, migrating configuration and linting to the Next 16-compatible TypeScript and ESLint 9 flat-config setup while preserving standalone deployment and the runtime API proxy. Supporting type fixes, dependency lockfile updates, documentation, and contributor workflow changes are included.

Flow diagram for the Next.js 16 frontend validation workflow

flowchart LR
    Install[npm ci] --> Typecheck[npm run typecheck]
    Install --> Lint[npm run lint]
    Install --> Build[npm run build]
    Build --> Smoke[Standalone server smoke test]

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File-Level Changes

Change	Details	Files
Upgrade the frontend framework, React runtime, and related ecosystem packages to Next.js 16.	Move Next.js from 14.2.35 to 16.3.8 and React/React DOM from 18.2 to 19.3. Update NextAuth, Lucide, ESLint, and Next ESLint configuration packages for compatibility. Refresh the lockfile without peer-dependency overrides.	frontend/package.json frontend/package-lock.json
Replace deprecated Next linting and adapt the project to ESLint 9 flat configuration.	Replace next lint with the ESLint CLI and add a dedicated typecheck script. Remove the legacy ESLint config and define Next Core Web Vitals and TypeScript flat presets. Keep five newly surfaced React/framework rules at warning severity while preserving errors for other violations.	frontend/.eslintrc.json frontend/eslint.config.mjs frontend/package.json
Migrate and simplify Next.js configuration for Next 16 while preserving deployment behavior.	Convert next.config.js to typed next.config.ts and remove unsupported configuration. Retain standalone output and unoptimized images; keep the runtime API proxy in the route handler so BACKEND_URL is resolved per request. Update generated Next type references and TypeScript inclusion settings.	frontend/next.config.js frontend/next.config.ts frontend/app/api/v1/[...path]/route.ts frontend/next-env.d.ts frontend/tsconfig.json
Resolve React 19 and stricter TypeScript compatibility issues in application and test code.	Use typed next-intl translators instead of any in outfit-card helpers. Convert empty props interfaces to type aliases and make a test variable immutable.	frontend/components/outfits/outfit-card.tsx frontend/components/ui/input.tsx frontend/components/ui/textarea.tsx frontend/tests/upload-manager.test.ts
Update contributor and user documentation for the new frontend workflow and runtime requirements.	Document Node.js 20.9+ for frontend development, with CI and Docker using Node.js 24. Direct contributors to the new npm run typecheck command.	CONTRIBUTING.md README.md

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