Allow the conversation content to grow wider

[RobvanderMost-TomTom]

The conversation content currently has a fixed maximum width. Increasing the window does not increase the width of the conversation. Instead the conversation is centered in a mostly empty screen.

Some generated content can benefit from a wider view. Especially generated code, which is now shown with its own horizontal scrollbar, even though there should be enough space to show the content without a scrollbar.