feat(tui): add assistant message steps counter to sidebar

[kajogo777]

Summary

Adds a Steps counter to the Context section of the sidebar, showing the number of assistant message rounds/steps in the current session.

Changes

• Add assistant_message_count field to AppState
• Increment counter when new assistant messages are created (in both streaming and non-streaming handlers)
• Display Steps row in Context section before Tokens with same color (White)
• Update context section height from 5 to 6 to accommodate the new row

Screenshot

The sidebar now shows:

▾ Context
    Steps                    N/A (or count)
    Tokens                   N/A (or usage)
    Model                    ...
    Provider                 ...