another step

client/eslint.config.mjs

@@ -9,6 +9,31 @@
 import next from 'eslint-config-next';
 import prettier from 'eslint-config-prettier/flat';
 
+// Unused variables/imports are dead code. eslint-config-next already enables
+// `@typescript-eslint/no-unused-vars` at 'warn'; we raise it to 'error' so
+// `npm run check` fails on dead code instead of merely warning. We patch the
+// severity *in place* on next's own config objects rather than adding a separate
+// override object, because in flat config a rule can only be referenced from a
+// config object that registers its plugin — and the `@typescript-eslint` plugin
+// is registered inside next's objects, not ours. Prefix a name with `_` to opt
+// out (intentionally-unused args, catch bindings, rest-spread siblings).
+const NO_UNUSED_VARS = [
+  'error',
+  {
+    args: 'after-used',
+    argsIgnorePattern: '^_',
+    varsIgnorePattern: '^_',
+    caughtErrorsIgnorePattern: '^_',
+    ignoreRestSiblings: true,
+  },
+];
+
+const nextWithStrictUnusedVars = [...next].map((entry) =>
+  entry?.rules?.['@typescript-eslint/no-unused-vars']
+    ? { ...entry, rules: { ...entry.rules, '@typescript-eslint/no-unused-vars': NO_UNUSED_VARS } }
+    : entry
+);
+
 /** @type {import('eslint').Linter.Config[]} */
 const config = [
   // Things ESLint should never look at.
@@ -16,17 +41,15 @@ const config = [
     ignores: ['.next/**', 'out/**', 'coverage/**', 'node_modules/**', 'next-env.d.ts'],
   },
 
-  // Next.js + TypeScript + React + import/a11y rule sets.
-  ...next,
+  // Next.js + TypeScript + React + import/a11y rule sets, with no-unused-vars
+  // raised to error (see NO_UNUSED_VARS above).
+  ...nextWithStrictUnusedVars,
 
   // Turn off every stylistic rule that would fight Prettier. Keep this last
   // among the rule-providing entries so it wins. Formatting is owned by
   // Prettier (`npm run format`), never by ESLint.
   prettier,
 
-  // Project-specific rule overrides go here, e.g.:
-  //   { rules: { 'react/jsx-key': 'error' } }
-  //
   // NOTE: `import/no-cycle` is intentionally NOT enabled. On this toolchain the
   // eslint-plugin-import TypeScript resolver bundled by eslint-config-next 16
   // throws "invalid interface loaded as resolver" for that rule, and it cannot