refactor(frontend): improve type safety and migrate to ESLint 9

Migrate the frontend to ESLint 9, add `eslint.config.mjs`, and replace various `any` types with more specific or safer alternatives across the codebase.

- Update `package.json` with new ESLint and TypeScript-ESLint dependencies
- Replace `any` with `Record<string, unknown>` or specific types in `TemplatePicker`, `Editor`, and `FileBrowser`
- Improve error handling in `TeamSettings` and `Templates` using type guards
- Add explicit type imports for Excalidraw elements in `Editor.tsx`
- Refactor `Modal.tsx` for cleaner conditional logic
- Add `check` script to `package.json` for type checking

frontend/eslint.config.mjs

@@ -0,0 +1,19 @@
+import tseslint from 'typescript-eslint';
+import pluginReact from 'eslint-plugin-react';
+
+export default tseslint.config(
+  { ignores: ['dist/', 'node_modules/', 'dist-test/', '*.config.*', 'eslint.config.mjs', 'e2e/', 'playwright/'] },
+  tseslint.configs.recommended,
+  {
+    files: ['**/*.{ts,tsx}'],
+    plugins: { react: pluginReact },
+    languageOptions: {
+      parserOptions: { project: './tsconfig.json' },
+    },
+    rules: {
+      '@typescript-eslint/no-explicit-any': 'warn',
+      '@typescript-eslint/no-non-null-assertion': 'warn',
+      'react/react-in-jsx-scope': 'off',
+    },
+  }
+);