feat(i18n): src/content/learn/manipulating-the-dom-with-refs.md from English to Vietnamese

[KotonoSora]

🇻🇳 Vietnamese Translation: Manipulating the DOM with Refs

Hi! I'm contributing as a volunteer translator for vi.react.dev, and this PR adds the Vietnamese translation for the "Manipulating the DOM with Refs" lesson.

What's included

• Vietnamese translation of manipulating-the-dom-with-refs.md
• Preserved all code examples and markdown formatting
• Kept terminology consistent with existing translations

Changes

• ✅ Fully translated the guide covering DOM manipulation using refs, accessing DOM nodes, managing ref callbacks, and best practices
• ✅ Maintained code blocks, links, Sandpack components and Challenge/Solution sections
• ✅ Verified all internal links and formatting render correctly

Looking forward to contributing more translations to the project! 🚀

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Tóm tắt những gì đã được dịch:

Tiêu đề và phần Intro:
"Manipulating the DOM with Refs" → "Thao tác DOM với Refs"

Các phần chính:

• "Getting a ref to the node" → "Lấy ref đến node"
• "Accessing another component's DOM nodes" → "Truy cập DOM nodes của component khác"
• "When React attaches the refs" → "Khi React attach refs"
• "Best practices for DOM manipulation with refs" → "Best practices cho thao tác DOM với refs"

Các ví dụ và phần phụ:

• "Example: Focusing a text input" → "Ví dụ: Focus vào text input"
• "Example: Scrolling to an element" → "Ví dụ: Scroll đến một element"
• "How to manage a list of refs using a ref callback" → "Cách quản lý danh sách refs sử dụng ref callback"
• "Exposing a subset of the API with an imperative handle" → "Expose một tập con của API với imperative handle"
• "Flushing state updates synchronously with flushSync" → "Flushing state updates đồng bộ với flushSync"

Phần Challenges:

• "Play and pause the video" → "Play và pause video"
• "Focus the search field" → "Focus search field"
• "Scrolling an image carousel" → "Scrolling image carousel"
• "Focus the search field with separate components" → "Focus search field với các components riêng biệt"

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Name	Status	Preview	Comments	Updated (UTC)
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