Allow-list (whitelist) for keyword-based message blocking

[Jssmme]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the request will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.

Feature description

The app currently supports blocking messages that contain specific keywords, but there's no way to define exceptions. If a legitimate message happens to contain a blocked keyword (for example, a bank notification that includes a word also used by spam messages), it gets blocked along with the actual spam, with no way to let it through.

Why do you want this feature?

Add a second list of keywords/phrases that work as a whitelist. When an incoming message matches an allow-list entry, it should bypass the keyword blacklist check entirely and be delivered normally, even if it also matches a blacklisted keyword. This would let users write broader, more effective blacklist rules (e.g. regex-based) without worrying about false positives on messages they actually want to receive.

Additional information

Some other SMS filtering tools (e.g. NekoSMS, CC SMS Blocker) implement this blacklist + whitelist combination, with whitelist rules taking priority over blacklist rules. Happy to attempt a PR for this once the direction is agreed on.