Improve PR header layout on narrow screens to prioritize description

[rebornix]

On smaller screens or in narrow editor tabs, the PR detail view moves the Reviewers, Assignees, Labels, and Project section to the top. This pushes the PR description further down, even though it’s often the most relevant content for reviewers and contributors.

It would be helpful to reconsider the layout for narrow viewports so that the description remains prominent and accessible without unnecessary scrolling.

I like how the GitHub mobile app fine-tunes the layout for narrow screens, though I’m not a fan of how it moves all the actions into floating buttons, which makes it harder to understand what each one does at a glance.

I wonder if we can consider collapsing the Reviewers/Assignees/Labels/Project section

[mrleemurray]

Agreed - that is a lot of unused space. Let's fix that :)

[mrleemurray]

Here is a proposal for a collapsible metadata section under the header section

I have also tidied up the metadata layout to more closely align with GH, which also reduces the amount of vertical space required.

[alexr00]

I wanted to do something like what GitHub does with a collapsed, readonly, narrow view:

Lee's proposed this, which looks way better than mine!