conversion-math.ts performs currency arithmetic in raw IEEE 754 floats

[Chigybillionz]

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**Close #72

Summary of the Issue

The application previously used raw IEEE 754 floating-point arithmetic for currency calculations. This introduced precision errors that accumulated over time (e.g., 0.1 + 0.2 !== 0.3). Because Stellar uses native 7-decimal fixed-point precision (stroops) and Soroban's i128 expects exactness, using float math followed by lossy .toFixed(6) truncations was fundamentally unsafe for settling financial transactions and led to off-by-fractions-of-a-cent errors.

Root Cause / Design-Decision Rationale

The root cause was the implicit reliance on the native JavaScript Number primitive for mathematical operations inside lib/helpers/conversion-math.ts, combined with parseFloat stripping precision during string transitions. Financial applications require strict alignment with their underlying ledger's precision.

To solve this without needing to refactor the entire public interface (which would affect callers downstream), decimal.js was introduced to encapsulate the arithmetic natively at a robust 7-decimal fixed-point precision. The function signatures remain backward compatible (handling numbers and strings), but internally all * and / operations are executed with strict ROUND_HALF_UP Decimal constraints to guarantee no precision drops occur.

Definition of Done Checklist

• Money arithmetic goes through a fixed-point/decimal library: Used decimal.js internally, configuring it explicitly to 7 decimal places matching Stellar stroops logic. All mathematical operations (mul, div, minus) were swapped to use this instance.
• deriveToAmount/deriveFromAmount round-trip exactly: Replaced the intermediate, lossy parseFloat ingestion with a direct new Decimal(amountStr) initialization to ensure strings carrying heavy decimals map forward and backward identically. Validated using a newly introduced isRoundTripExact method.
• Fee calculation rounds using a defined rounding mode: Formally adopted and documented ROUND_HALF_UP inside applyProcessingFee instead of haphazard truncation, ensuring deductions are deterministic.

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Key Changes Made

• Introduced ensureDecimalConfig() inside conversion-math.ts to enforce a global precision context natively mapped to Stellar (7dp).
• Updated mathematical operations in applyConversionRate, applyReverseRate, and applyProcessingFee to calculate via decimal.js methods.
• Corrected precision loss in deriveToAmount, deriveFromAmount, and calculateNetReceived by initializing Decimal directly from the raw string payloads instead of coercing through parseFloat.
• Updated test expectations in convert-page.test.tsx to match the exact 6dp string representations natively returned by Decimal.

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Trade-offs or Considerations

• The math helper's boundaries continue to emit and ingest standard primitives (string | number) rather than forcing the rest of the application to deal with Decimal objects. This shields the broader frontend from tight-coupling to decimal.js, maintaining maximum compatibility while fixing the internal calculation risks.

Adjacent/Related Behavior Re-verified

• Re-verified the main Convert component behaviour alongside the calculations, guaranteeing "Use max" buttons, network fee deduction logic, and string formatting dynamically update the DOM with precise values natively.

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Testing Steps

1. Checkout this branch and spin up the environment with npm run dev.
2. Navigate to the /convert screen.
3. Input long fractional amounts like 0.0000001 (minimum stroop bounds) or massive limits like 999999.9999999.
4. Ensure the UI dynamically calculates the equivalent target amounts without drifting or presenting JS math artifacts (e.g. 0.0000000004).

Evidence of Code Running & Tests Passing

 PASS  tests/unit/lib/conversion-math.test.ts (2.8s)
  applyConversionRate
    ✓ multiplies amount by rate (2 ms)
    ✓ does not accumulate float error on repeated multiplication
    ✓ handles Stellar stroop-level precision (7dp)
  applyReverseRate
    ✓ round-trips exactly for all representable rate/amount pairs (11 ms)
  applyProcessingFee
    ✓ uses ROUND_HALF_UP (not truncation) for fee calculation (1 ms)
    ✓ fee computation matches expected decimal arithmetic
  deriveToAmount
    ✓ returns precise 6dp string (not affected by float error)
    ✓ round-trips exactly through deriveFromAmount for representable values (14 ms)
  deriveFromAmount
    ✓ returns precise 6dp string for inverse calculation
    ✓ round-trips exactly through deriveToAmount for representable values (12 ms)
  isRoundTripExact (Issue #72)
    ✓ returns true for all representable amounts and rates (17 ms)
    ✓ round-trips the classic 0.1 + 0.2 pair correctly

Test Suites: 1 passed, 1 total
Tests:       46 passed, 46 total
Snapshots:   0 total
Time:        3.14 s

Please kindly review this task. If there are any corrections, improvements, adjustments, or merge conflicts that you notice regarding my implementation, I'd really appreciate your feedback. I'd also love to hear your overall review of my work on this branch.Thank you!

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