Graphical Header improvements

[nhojb]

Minor improvements to the graphical header:

• Adds some padding at the bottom of the header so that prompt text is not immediately adjacent to the bottom text in the header. This was mostly a problem in the viewport mode.
• Improves alignments of the header icon.

Checklist

• I've read the README's Contributing section.
• I've filed a feature request/discussion for a new feature.
• My code follows the project style.
• I've added tests where applicable.
• I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.
• I've reviewed all code in PR myself and will vouch for its quality.

[xenodium]

Thanks for the PR! Unfortunately, the current graphical layout is a bit of a stretch (via svg) and has some hardcoded logic that doesn't seem to scale well across folks setups.

For example, what you see today works fairly well for my setup (check out the screenshots over at https://xenodium.com/agent-shell-0-25-updates). However, I appreciate this may not work well for everyone's setups and is fragile. I've just applied #275 and I'm getting the following layouts (shell and compose)

We may need to revisit the layout logic and see if we can remove some of the assumptions and hardcoded values in favor of more dynamic/derived ones. I've put out an experimental PR at #276 Could you give that a try and see if that works better for your setup?