fix: use BigInt to prevent overflow when parsing large token amounts

[agentmila]

Bug

The token amount parser in account-data.ts uses parseInt() to parse raw token amounts. When token amounts exceed Number.MAX_SAFE_INTEGER (~9.0 × 10¹⁵), JavaScript loses precision, producing incorrect or negative values.

This causes the overflow issue reported in #280, where a minting transaction shows a negative amount instead of the correct minted value.

Root Cause

parseInt(tokenAmount) returns a JavaScript Number, which only has 53 bits of integer precision. Token amounts in lamports or with high decimals can easily exceed this.

Fix

Use BigInt() for intermediate arithmetic, converting to Number only after dividing to a safe range:

• For 0-decimal tokens: Number(BigInt(tokenAmount))
• For tokens with decimals: split into whole + remainder using BigInt division, then combine as Number

This preserves precision for large amounts while maintaining the existing number type interface.

Fixes #280

[vercel]

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