Add ESLint + pre-commit hook and fix the issues it surfaced

eslint.config.js

@@ -0,0 +1,54 @@
+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(
+  // Ignore build output and generated artifacts.
+  { ignores: ['dist', 'dev-dist', 'coverage'] },
+
+  js.configs.recommended,
+
+  // Type-aware linting for the TypeScript sources only.
+  {
+    files: ['**/*.{ts,tsx}'],
+    extends: [...tseslint.configs.recommendedTypeChecked],
+    languageOptions: {
+      ecmaVersion: 2022,
+      globals: { ...globals.browser, __APP_VERSION__: 'readonly' },
+      parserOptions: {
+        // Type-aware linting: powers no-floating-promises, no-misused-promises, etc.
+        projectService: true,
+        tsconfigRootDir: import.meta.dirname,
+      },
+    },
+    plugins: {
+      'react-hooks': reactHooks,
+      'react-refresh': reactRefresh,
+    },
+    rules: {
+      ...reactHooks.configs.recommended.rules,
+      'react-refresh/only-export-components': ['warn', { allowConstantExport: true }],
+      // Allow intentionally-unused args/vars when prefixed with _ (matches tsconfig intent).
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        { argsIgnorePattern: '^_', varsIgnorePattern: '^_' },
+      ],
+    },
+  },
+
+  // Tests exercise looser patterns and run in Node.
+  {
+    files: ['**/*.test.{ts,tsx}'],
+    languageOptions: { globals: { ...globals.node } },
+  },
+
+  // Plain JS config files (this file, etc.) are not part of the TS project —
+  // run them through the untyped ruleset only.
+  {
+    files: ['**/*.js'],
+    extends: [tseslint.configs.disableTypeChecked],
+    languageOptions: { globals: { ...globals.node } },
+  },
+);