build(deps): migrate to eslint 9 flat config

eslint.config.mjs

@@ -0,0 +1,52 @@
+import js from '@eslint/js'
+import tseslint from 'typescript-eslint'
+import reactHooks from 'eslint-plugin-react-hooks'
+import reactRefresh from 'eslint-plugin-react-refresh'
+import globals from 'globals'
+
+export default tseslint.config(
+  { ignores: ['dist', 'dist-electron', 'release'] },
+  {
+    files: ['**/*.{ts,tsx}'],
+    extends: [js.configs.recommended, ...tseslint.configs.recommended],
+    languageOptions: {
+      ecmaVersion: 'latest',
+      sourceType: 'module',
+      globals: { ...globals.browser, ...globals.es2020, ...globals.node },
+    },
+    plugins: {
+      'react-hooks': reactHooks,
+      'react-refresh': reactRefresh,
+    },
+    rules: {
+      ...reactHooks.configs.flat['recommended-latest'].rules,
+      'react-refresh/only-export-components': [
+        'warn',
+        { allowConstantExport: true },
+      ],
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        { argsIgnorePattern: '^_', varsIgnorePattern: '^_' },
+      ],
+      'no-unused-vars': 'off',
+      // React Compiler-readiness rule (added in eslint-plugin-react-hooks v7);
+      // this project doesn't use the Compiler, and it flags the standard
+      // "reset state, then kick off an async load" effect pattern used
+      // throughout src/components as an error.
+      'react-hooks/set-state-in-effect': 'off',
+    },
+  },
+  {
+    // Electron main/preload process — no DOM/window globals.
+    files: ['electron/**/*.ts'],
+    languageOptions: {
+      globals: { ...globals.node },
+    },
+  },
+  {
+    // Context/toggle colocated with its hooks/constants — Fast Refresh
+    // granularity is not worth splitting these files for.
+    files: ['src/theme.tsx', 'src/components/ThemeToggle.tsx'],
+    rules: { 'react-refresh/only-export-components': 'off' },
+  },
+)