feat: add comprehensive backup functionality for vault operations

[Lucklyric]

Summary of Changes:

• Added backup prompts when saving/modifying vault files (default: Yes)
• Added backup prompt for GPG encryption (default: Yes)
• ASCII armor prompt now defaults to Yes
• Improved backup file naming to preserve extensions
• Added overwrite protection for existing files

Detailed Changes:

1. Vault Backup System (service.rs):

   • Added create_backup() method that creates timestamped backups
   • Integrated backup prompt in save_vault() with Y default
   • Backup format: vault.toml.backup.{timestamp}

2. GPG Improvements (gpg.rs):

   • Updated encrypt_vault() to handle .toml.gpg/.toml.asc extensions
   • Added overwrite prompts for existing GPG files
   • Improved backup_vault() to create .gpg.backup format for GPG files
   • Fixed extension handling in decrypt_vault()

3. User Prompts (utils.rs):

   • Added prompt_yes_no() utility for consistent Y/n prompting
   • Used throughout for backup and overwrite confirmations

4. CLI Updates (cli.rs):

   • Updated gpg_encrypt to use prompt_yes_no with Y default
   • Removed redundant backup code (now handled by GPG module)

5. Interactive Mode (interactive.rs):

   • Changed ASCII armor prompt to Y default
   • Changed backup prompt to Y default
   • Used prompt_yes_no for consistency

File Naming Conventions:

• Regular backups: vault.toml.backup.20250721_152607
• GPG backups: vault.toml.gpg.backup.20250721_152607
• GPG encrypted: vault.toml.gpg (binary) or vault.toml.asc (ASCII)

All changes maintain backward compatibility while improving user safety through better backup defaults and overwrite protection.