Add UX/UI empty-states section to qa-dev-design considerations doc

[georgekarrv]

Related user story

As a g-software designer/engineer/QA, I want explicit guidance on empty states so that we catch missing UX coverage before features ship.

Task

Add a new UX/UI section to qa-dev-design considerations.md covering empty states. Section should call out:

• When an empty state is required.
• What content/CTAs the empty state needs.
• How QA should verify it during review.

Condition of satisfaction

• New UX/UI section (including empty states guidance) merged into qa-dev-design considerations.md.
• Done by 2026-06-29 (next retro).

[georgekarrv]

#47917