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

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

core/platform/tsdown.config.ts

@@ -3,6 +3,7 @@ import { sharedConfig } from '@robonen/tsdown';
 
 export default defineConfig({
   ...sharedConfig,
+  tsconfig: './tsconfig.src.json',
   entry: {
     browsers: 'src/browsers/index.ts',
     multi: 'src/multi/index.ts',