fix(utils): validate fractionDigits in getRoundedAmount to prevent RangeError

[jaffarkeikei]

What changed? Why?

getRoundedAmount now validates fractionDigits parameter to prevent RangeError crashes. toFixed() only accepts values 0-100.

Notes to reviewers

Without validation, passing negative or >100 values causes:

getRoundedAmount('123.456', -1)  // RangeError: toFixed() digits argument must be between 0 and 100

Fix returns original balance for invalid inputs.

How has it been tested?

Manual testing with edge cases:

• Negative fractionDigits: returns original balance

• 100 fractionDigits: returns original balance

• Valid range (0-100): works as expected

[cb-heimdall]

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