fix: AEAD stream finalization, secure wipe, and password confirmation

[vylarion]

1. Fix AEAD stream finalization (security) The encrypted stream never called encrypt_last/decrypt_last, only encrypt_next/decrypt_next. This meant the stream was never cryptographically sealed — an attacker could silently truncate the ciphertext without detection.

EncryptedWriter now calls encrypt_last() for the final chunk via an explicit finalize() method. DecryptedReader uses a lookahead pattern to detect the final encrypted chunk and decrypt it with decrypt_last().

2. Implement real secure wipe (security) The --wipe flag used fs::remove_file() which only unlinks the file — data remains on disk and is forensically recoverable. Now overwrites file contents with cryptographically random data and calls sync_all() before unlinking. Directories are recursively wiped file-by-file.

3. Add password confirmation on pack (UX) pack now prompts for password confirmation to prevent typos that would make data irrecoverable, which is especially critical when combined with --wipe.