[FEATURE] Support Batch Decryption from File

[Lucaas-o]

Problem
Users must enter each ciphertext manually, which is inefficient for multiple inputs.

Solution
Add a menu option (e.g., 5. Batch Decrypt from File):

• Read lines from a file (e.g., input.txt).
• Run automated decryption on each line.
• Save results to batch_results.txt.

Alternatives
Could process a comma-separated list in one prompt instead.

Additional Context

• Example input.txt:
  Khoor123
  SGVsbG8=
• Use Python’s open() and loop through lines.