Reject invalid Shamir share inputs

[KingParmenides]

Summary

• Reject non-decimal threshold/total input before generating Shamir shares.
• Parse validated inputs explicitly as base-10 integers.
• Enforce a minimum threshold of 2 so invalid values cannot reach splitMnemonic.

This implements a direct web-tool validation fix for the invalid-threshold behavior reported in #6 and bitaps-com/jsbtc#48. Credit for the original report remains with that reporter; this PR is the implementation patch.

Verification

• git diff --check

Bounty payout address for accepted implementation work: 39Q34P8A7g375yqEr8buvNJkUbRgKfbKQZ