feat(i18n): src/content/learn/referencing-values-with-refs.md from English to Vietnamese

[KotonoSora]

🇻🇳 Vietnamese Translation: Referencing Values with Refs

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

What's included

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

Changes

• ✅ Fully translated the guide covering how to use refs to reference values that don't trigger re-renders, including practical examples like building a stopwatch
• ✅ 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: "Referencing Values with Refs" → "Tham chiếu giá trị với ref"

Các phần chính:

• "Adding a ref to your component" → "Thêm ref vào component của bạn"
• "Example: building a stopwatch" → "Ví dụ: xây dựng đồng hồ bấm giờ"
• "Differences between refs and state" → "Sự khác biệt giữa ref và state"
• "When to use refs" → "Khi nào nên dùng ref"
• "Best practices for refs" → "Best practices cho ref"
• "Refs and the DOM" → "Ref và DOM"

Phần Challenges:

• "Fix a broken chat input" → "Sửa input chat bị lỗi"
• "Fix a component failing to re-render" → "Sửa component không re-render"
• "Fix debouncing" → "Sửa debounce"
• "Read the latest state" → "Đọc state mới nhất"