Security improvement: PINs are now stored as salted SHA-256 hashes
instead of plain text.
- PinSecurity.kt: Hash/verify utilities with salt
- AdminActivity: Uses hashed PINs for storage and verification
- Backward compatible: verifies against legacy plain text PINs
- PIN field shows placeholder when hashed (enter new to change)
- PinSecurityTest.kt: Unit tests for hashing logic
Format: 'base64salt:base64hash' stored in DataStore
