chore(platform): eslint/tsconfig migration + browser utils

Migrate to eslint flat config and composite tsconfig; browser/multi updates
(hideOthers, focusScope).

core/platform/eslint.config.ts

@@ -0,0 +1,9 @@
+import { base, compose, imports, stylistic, typescript } from '@robonen/eslint';
+
+export default compose(base, typescript, imports, stylistic, {
+  name: 'platform/overrides',
+  files: ['src/multi/global/index.ts'],
+  rules: {
+    'unicorn/prefer-global-this': 'off',
+  },
+});