fix: messages not sending to joined channel from public or private chat

[whflima]

Closes #234

Description

This PR addresses an issue where:

• Messages sent to a channel (from public or private chats) that the user has already joined are not being delivered.

The issue stemmed from missing or incorrect handling of channel switching logic.

Why this matters

• Ensures that users can send messages to channels they’ve already joined.

What’s changed

• If a channel is not joined, the app now calls joinChannel(channel) and exits the current iteration using return@forEach.

• If the channel is already joined, the app calls switchToChannel(channel) before sending the message.

🔍 How to Test

• Open the app.
• Join any channel. Example: #test_channel Join in channel
• From a public or private channel, send a message to the joined channel. Example: #test_channel Second message
• Verify that the message is successfully delivered and appears in the target channel.

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