Add ability to backup wallet shares to files and restore from backups:
- Add ShareBackup data class in Models.kt for backup format
- Add exportShareBackup() and importShareBackup() in TssRepository
- Add export/import state and methods in MainViewModel
- Add file picker integration in MainActivity using ActivityResultContracts
- Add import FAB button in WalletsScreen
- Export saves as .tss-backup file with address and timestamp in filename
- Import validates backup format and checks for duplicate wallets
The backup file contains all necessary data to restore a wallet share:
sessionId, publicKey, encryptedShare, threshold, partyIndex, address.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
hailin
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