Add timezone masking

In the situation where a timezone is known, it would be good if the tool could optionally mask out other time zones so the user can focus on the candidate areas in the matching timezone.

Start by locating the command-line entry point and the existing timezone or candidate-area matching logic. Add an optional way to mask areas in other time zones when a timezone is known, and verify that the default behavior remains unchanged and only matching-timezone candidates are shown when enabled.

Written by the indexing model from the issue text.