feat(channel): add Mattermost channel

[longkeyy]

Summary

• add built-in Mattermost channel plugin
• connect through Mattermost REST API and WebSocket events
• harden WebSocket TLS/timeout handling and edit streamed replies in place

Tests

• cargo fmt --all -- --check
• cargo clippy -p aionui-channel --features mattermost -- -D warnings
• cargo test -p aionui-channel --features mattermost
• cargo build --release -p aionui-app
• just push