Add support for inline html and html pane in deck

### What is the problem

Sometimes the code is not enough, neither does flows. Sometimes we actually need to go nuts and understand deeply. For example, I was using deck and it explained me that how there are two conflicting rows, and to get my head wrapped around there I needed to read that our twice of thrice, what I wish is that, either or and

1. We can someway display a small `html` block in the prose, like if I would have that support, the agent could have drawn two rows and animated them, how would the merge look like, by taking an example.
2. Or it has a dedicated html pane that it renders, where it can choose freedom to explain you by example what might be happening.

That's the case, flows need to be used for specific things, while html is used for specific things, where flows fail, where we actually need that freedom, and that freedom I think should be limited, very minimal allowed text, because prose control that, mostly for crafting or say creating a graph or something. Specific to visualize.

### What you had in mind

_No response_

The issue names no files, tests, or entry points. First map the existing prose and flow rendering paths, then clarify whether inline HTML, a dedicated HTML pane, or both are in scope; completion criteria are not specified.

Written by the indexing model from the issue text.