refactor(cleanup): consolidate hand-rolled Stellar StrKey/CRC16 decoding and base32 with the Stellar SDK

[cypriannwokolo2-creator]

Summary

auth/service.go and totp/service.go hand-roll StrKey/CRC16/base32 decoding instead of using the Stellar SDK's keypair/StrKey. This duplicates and risks diverging from the canonical implementation.

Acceptance Criteria

• Replace hand-rolled decoding with SDK equivalents
• Verify all existing encoded data still decodes identically
• Tests

Complexity

High — must be backward-compatible with stored/encoded data.

[AdEmOnD07]

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I would like to work on this issue
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[simplex001]

@simplex001 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

hello maintainer, i'd love to work on this issue! please assign me. I have reviewed the issue details and fully understand the requirements

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