fix(components): [watermark] mutationObserver should work

[Fuphoenixes]

closed #14819

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Description

🤖 Generated by Copilot at 7b8a093

Enhance watermark component to handle dynamic elements. Add subtree and childList options to MutationObserver in watermark.vue.

Related Issue

Fixes #14819 .

Explanation of Changes

🤖 Generated by Copilot at 7b8a093

• Add two options to the MutationObserver constructor to monitor dynamic changes in the target container (link)

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[github-actions]

[github-actions]

Hello @Fuphoenixes, thank you for contributing to element-plus, please see our guideline to see how to make contribution

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=14822
Please comment the example via this playground if needed.

[ryuhangyeong]

@TomatoDroid

Could you consider this review?

[Fuphoenixes]

这个pr可以合了吗？水印组件无法防篡改应该算是这个组件比较严重的问题 @btea @ryuhangyeong @TomatoDroid

[tenglongwentian]

为什么图片和文字水印不能组合使用？当图片水印存在的时候，文字水印就看不到。无法共存