fix: clear channel messages on reset

[elian1780]

Closes 422
When clearMessages is called, it now also clears the channelMessages in the state, ensuring that messages from specific channels are also removed.

Description

Checklist

• I have read the contribution guidelines: https://github.com/permissionlesstech/bitchat-android?tab=readme-ov-file#contributing
• I have performed a self-review of my code

• I have mentioned the corresponding issue and the relevant keyword (e.g., "Closes: #xy") in the description (see https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
• If it is a core feature, I have added automated tests

[callebtc]

thank you!