fix(react-sdk): correct sign handling in parseAssetAmount / formatAssetAmount

[batuhankocyigit]

fix(react-sdk): correct sign handling in parseAssetAmount / formatAssetAmount for negative amounts

The bug

formatAssetAmount / parseAssetAmount (exported from @miden-sdk/react, src/utils/amounts.ts) mishandle negative values.

parseAssetAmount splits the input on . and combines the whole and
fraction parts as BigInt(whole) * factor + BigInt(fraction). When the
input is negative, the minus sign only affects the whole part, not the
combined magnitude:

parseAssetAmount("-5.25", 2) // => -475n, expected -525n
parseAssetAmount("-0.5", 2)  // => 50n,   expected -50n (sign lost entirely,
                              //           since BigInt("-0") === 0n)

formatAssetAmount has the mirror problem: dividing/modding a negative
BigInt by a positive factor yields a negative remainder, which gets
concatenated after the decimal point, producing an invalid string:

formatAssetAmount(-525n, 2) // => "-5.-25", not a valid number at all

Both functions are part of the package's public API surface
(export { formatAssetAmount, parseAssetAmount } from "./utils/amounts" in
src/index.ts) and formatAssetAmount is also used internally in
utils/notes.ts to render note amounts, so any consumer surfacing a
negative amount (e.g. a balance delta, a signed transfer amount, a refund)
gets silently wrong output today — no throw, no warning.

The fix

Extract the sign once up front, operate on the absolute magnitude, and
re-apply the sign to the final result in both functions. Minimal, symmetric
change, no public API/signature change.

Testing

• Added regression tests for both functions covering negative whole
  numbers, negative fractional amounts, and the "-0.x" edge case, plus a
  round-trip test (parseAssetAmount → formatAssetAmount → same string)
  for negative inputs.
• Ran typecheck, lint, and the full unit test suite locally — all green
  (853/853 passing, 845 existing + 8 new).

Notes for reviewers

• No behavior change for non-negative amounts (existing test suite passes
  unmodified).
• I did not touch any other files; this is scoped to the one utility module.