fix: improve modal layout and theme bootstrap helpers

[wangjinpeng1235]

Summary

• improve the shared modal viewport layout so dialogs stay centered and avoid duplicated headers
• document the shared theme bootstrap helpers and restore script options for host apps
• keep the related UI theme tests aligned with the updated bootstrap behavior

Validation

• pnpm --filter @h-ai/ui lint
• pnpm --filter @h-ai/ui test -- tests/theme-config.test.ts