feat(i18n): translate src/content/learn/extracting-state-logic-into-a-reducer.md from English to Vietnamese

[KotonoSora]

🇻🇳 Vietnamese Translation: Extracting State Logic into a Reducer

Hi! I'm contributing as a volunteer translator for vi.react.dev, and this PR adds the Vietnamese translation for the "Extracting State Logic into a Reducer" lesson.

What's included

• Vietnamese translation of extracting-state-logic-into-a-reducer.md
• Preserved all code examples and markdown formatting
• Kept terminology consistent with existing translations

Changes

• ✅ Fully translated the guide covering state management with reducers, useReducer Hook, and refactoring from useState
• ✅ 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:
"Extracting State Logic into a Reducer" → "Trích xuất State Logic vào một Reducer"

Các phần chính:

• "Consolidate state logic with a reducer" → "Hợp nhất logic state với reducer"
• "Comparing useState and useReducer" → "So sánh useState và useReducer"
• "Writing reducers well" → "Viết reducer tốt"
• "Writing concise reducers with Immer" → "Viết reducer ngắn gọn với Immer"

Ba bước thực hiện:

• "Step 1: Move from setting state to dispatching actions" → "Bước 1: Di chuyển từ setting state sang dispatching action"
• "Step 2: Write a reducer function" → "Bước 2: Viết một function reducer"
• "Step 3: Use the reducer from your component" → "Bước 3: Sử dụng reducer từ component của bạn"

Phần Challenges:

• "Dispatch actions from event handlers" → "Dispatch action từ event handler"
• "Clear the input on sending a message" → "Xóa input khi gửi message"
• "Restore input values when switching between tabs" → "Khôi phục giá trị input khi chuyển đổi giữa các tab"
• "Implement useReducer from scratch" → "Triển khai useReducer từ đầu"

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