chore: migrate lint to eslint 9 flat config

Next 16 removed the `next lint` command, so `pnpm lint` was broken. Replace
.eslintrc.json with an ESLint 9 flat config (eslint.config.mjs) that spreads
eslint-config-next's native core-web-vitals flat config, and point the lint
script at `eslint .`.

Keep Next 16's newer react-hooks (v6) compiler-adjacent rules advisory (warn)
rather than blocking, since they flag working-but-non-optimal patterns across
the existing codebase. `pnpm lint` now passes (0 errors, warnings only).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/.eslintrc.json

@@ -1,6 +0,0 @@
-{
-  "extends": "next/core-web-vitals",
-  "rules": {
-    "react/no-unescaped-entities": "off"
-  }
-}

web/eslint.config.mjs

@@ -0,0 +1,32 @@
+import nextCoreWebVitals from 'eslint-config-next/core-web-vitals'
+
+// Next 16 removed `next lint`; eslint-config-next now ships a native ESLint 9
+// flat config, so we spread it directly (this replaces the old .eslintrc.json).
+const eslintConfig = [
+  ...nextCoreWebVitals,
+  {
+    rules: {
+      'react/no-unescaped-entities': 'off',
+      // Next 16 enables the newer react-hooks (v6) compiler-adjacent rules by
+      // default. They flag working-but-non-optimal patterns across this
+      // pre-existing codebase, so keep them advisory (warn) rather than
+      // blocking; tighten to error as components are refactored.
+      'react-hooks/set-state-in-effect': 'warn',
+      'react-hooks/purity': 'warn',
+      'react-hooks/static-components': 'warn',
+      'react-hooks/refs': 'warn',
+      'react-hooks/immutability': 'warn',
+      'react-hooks/incompatible-library': 'warn',
+    },
+  },
+  {
+    ignores: [
+      '.next/**',
+      'node_modules/**',
+      'test-results/**',
+      'playwright-report/**',
+    ],
+  },
+]
+
+export default eslintConfig

web/package.json

@@ -7,7 +7,7 @@
     "build": "next build",
     "vercel-build": "next build",
     "start": "next start",
-    "lint": "next lint",
+    "lint": "eslint .",
     "test": "vitest run",
     "test:watch": "vitest",
     "test:e2e": "playwright test",
@@ -63,6 +63,7 @@
     "node": ">=18.17.0"
   },
   "devDependencies": {
+    "@eslint/eslintrc": "^3.3.6",
     "@playwright/test": "^1.49.1",
     "@tailwindcss/postcss": "^4.3.2",
     "@testing-library/jest-dom": "^6.6.3",

web/pnpm-lock.yaml

@@ -138,6 +138,9 @@ importers:
         specifier: ^3.25.76
         version: 3.25.76
     devDependencies:
+      '@eslint/eslintrc':
+        specifier: ^3.3.6
+        version: 3.3.6
       '@playwright/test':
         specifier: ^1.49.1
         version: 1.61.1