feat(i18n): translate src/content/learn/managing-state.md from English to Vietnamese

[KotonoSora]

🇻🇳 Vietnamese Translation: Managing State

Hi! I'm contributing as a volunteer translator for vi.react.dev, and this PR adds the Vietnamese translation for the "Managing State" chapter.

What's included

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

Changes

• ✅ Fully translated the chapter covering React state management concepts and patterns
• ✅ Maintained code blocks, links, Sandpack components and LearnMore 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:
"Managing State" → "Quản Lý State"

Các phần chính:

• "Reacting to input with state" → "Phản ứng với đầu vào thông qua state"
• "Choosing the state structure" → "Chọn cấu trúc state"
• "Sharing state between components" → "Chia sẻ state giữa các component"
• "Preserving and resetting state" → "Bảo tồn và reset state"
• "Extracting state logic into a reducer" → "Trích xuất logic state vào reducer"
• "Passing data deeply with context" → "Truyền data sâu với context"
• "Scaling up with reducer and context" → "Mở rộng với reducer và context"

Phần YouWillLearn:

• Các bullet point về cách suy nghĩ về UI changes, cấu trúc state, lifting state up, state preservation/reset, reducer patterns, context usage, và scaling strategies

Phần "What's next?":

• "What's next?" → "Tiếp theo là gì?"

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