Fix: user balance decimal128

[Kirtan-pc]

📌 Overview

This PR fixes the duplicate balance field definition in backend/models/User.js where the field was declared twice with conflicting types — first as mongoose.Schema.Types.Decimal128 with a fromString('100000') default, immediately followed by type: Number with default: 100000. Mongoose silently used only the last definition, causing all balance operations to use raw JavaScript Number with floating-point precision instead of Decimal128, resulting in silent financial rounding errors.

🛠️ Type of Change

• ⛓️ Smart Contract (Solidity changes, Gas optimization)
• 💻 Frontend (UI/UX, React components, Tailwind)
• ⚙️ Backend (API routes, MongoDB schemas, Middleware)
• 📄 Documentation (README, Roadmap updates)
• 🧪 Testing (Hardhat tests, Jest/Vitest)

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🔗 Related Issue

Closes #828

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🧪 Testing & Verification

• Smart Contracts: npx hardhat test passed? (Yes/No/NA)
• Frontend: Verified on Mobile/Desktop responsiveness? (Yes/No/NA)
• Integration: Verified User.js loads without errors and balance uses Decimal128 type

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📸 Screenshots / Demos

N/A

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✅ PR Checklist

• My code follows the project's style guidelines.
• I have commented my code, particularly in complex areas (e.g., Smart Contract logic).
• I have updated the documentation accordingly.
• My changes generate no new warnings.

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💬 Additional Notes

The duplicate field was introduced during a previous merge conflict resolution that left both old (Number) and new (Decimal128) balance definitions in the schema. Removing the duplicate Number definition ensures all balance operations use consistent Decimal128 precision for accurate financial calculations across bids, fund transfers, and balance queries.

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