feat(ffi): expose NUT-27 mint backup

[thesimplekid]

Add WalletRepository FFI methods for deriving the backup public key, publishing mint backups, restoring mints, and fetching a backup preview. Also avoid duplicate mint URLs in backups and fix the example's NUT label.

Description

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Notes to the reviewers

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Suggested CHANGELOG Updates

CHANGED

ADDED

REMOVED

FIXED

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Checklist

• I followed the code style guidelines
• I ran just quick-check before committing
• If the Wallet API was modified (added/removed/changed), I have reflected those changes in the FFI bindings (crates/cdk-ffi)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.88%. Comparing base (a5cace5) to head (08fde41).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/cdk-ffi/src/wallet_repository.rs	0.00%	32 Missing ⚠️
crates/cdk/src/wallet/nostr_backup.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2153      +/-   ##
==========================================
- Coverage   71.91%   71.88%   -0.04%     
==========================================
  Files         356      356              
  Lines       74500    74532      +32     
==========================================
- Hits        53577    53574       -3     
- Misses      20923    20958      +35     

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[asmogo]

ACK

[cdk-bot]

Successfully created backport PR for v0.17.x:

• [Backport v0.17.x] feat(ffi): expose NUT-27 mint backup #2159