🤏 Improve theme for narrow screens

[agoose77]

This PR sets some flex styling to avoid shrinkage causing issues at intermediate widths.
This PR does not address the need to perhaps tweak the menu breakpoints / move the theme button into the ellipsis menu.

Existing:

• Unnecessarily wide search bar
• Stretched theme icon
• Warped action button(s)

New:

• Narrower search bar
• Non-growing/shrinking (square) theme icon
• Non-shrinking action button(s)

[changeset-bot]

🦋 Changeset detected

Latest commit: 767984b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 14 packages

Name	Type
@myst-theme/site	Patch
@myst-theme/providers	Patch
@myst-theme/frontmatter	Patch
@myst-theme/diagrams	Patch
@myst-theme/jupyter	Patch
@myst-theme/styles	Patch
@myst-theme/common	Patch
@myst-theme/icons	Patch
@myst-theme/search	Patch
@myst-theme/search-minisearch	Patch
@myst-theme/book	Patch
@myst-theme/article	Patch
myst-to-react	Patch
myst-demo	Patch

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[rowanc1]

Can you put screenshots in?!

[stevejpurves]

@agoose77 is wide screen behaviour unchanged? even when there are a good list of nav items in the topbar?

[agoose77]

@stevejpurves I've done some more testing on that, and I think it needs some work.

I think we need to start applying more conditional styling so that if users have nav items, we prefer compactness at intermediate sizes. Does anyone have any thoughts here? It's a bit of a feedback loop between nav and search at the moment.