fix: properly handle keyword blocking

[naveensingh]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

Implemented the new isContentBlocked() method from FossifyOrg/mmslib#2. This ensures that the received messages are checked for blocked keywords before they are saved to the system database.

Note: This could have been fixed easily by filtering out existing messages with blocked keywords at run time, but this way is more consistent with how messages from blocked numbers are dropped.

Tests performed

• Receiving MMS messages with text (notification + in-app refresh)
• Receiving MMS messages without text (notification + in-app refresh)
• Receiving MMS messages with blocked keywords
• Receiving MMS messages with previously blocked keywords unblocked
• Sending MMS messages including blocked keywords
• Sending MMS messages with or without text

I couldn't test it, but Aga-C and laubster have confirmed that this works.

Closes the following issue(s)

• Closes Message with Blocked Keyword Got Through #99

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• All checks are passing.