Use a responsive grid layout for the brainstorming cards

[elken]

Description of changes:

Having to awkwardly scroll sideways makes for a frustrating UX, so this PR aims to create a flexible grid of cards instead

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[dboyd13]

Thank you for your PR. The responsive grid layout you've proposed was originally considered, but we did not go in this direction as if one has a lot of cards under a section (e.g. assumptions) then those sections that are wrapped (e.g. mitigations) are moved out of view with no visual hint that they may exist, and are also a variable and unpredictable scroll depth. The current approach provides a visual indicator that there are other sections (horizontal scroll bar), and also are a predictable and fixed (short) scroll length away.

I will a few known point users towards this PR for folks to weigh-in with their feedback. I encourage others that may be reading this PR to do the same.

Here are some comparison screen recordings for those that don't want to have to build:

Current (main)
https://github.com/user-attachments/assets/2ab81814-8744-4550-bccd-7fee8fa15aff

Proposed (this PR)
https://github.com/user-attachments/assets/a6306278-f83b-4be9-8912-c734c4b9ee29

[elken]

We had a session with a few of us the other day and we all had the same feedback that the very long side-scrolling was very awkward and hard to jump around context.

But thank you for considering it 😄