fix(dropdown): normalize hover-only overlay menu styles

[wangdicoder]

Summary

• remove persistent selected-state visuals from Dropdown overlay menus and keep hover-only feedback
• align grouped popup item hover styling with the rest of dropdown submenu items
• document the normalized dropdown menu behavior and add a changeset

Verification

• pnpm exec stylelint packages/react/src/dropdown/style/index.scss
• pnpm exec prettier --check packages/react/src/dropdown/dropdown.tsx packages/react/src/dropdown/index.md packages/react/src/dropdown/index.zh_CN.md packages/react/src/dropdown/style/index.scss
• pnpm --filter @tiny-design/react test -- --runInBand packages/react/src/dropdown/tests/dropdown.test.tsx