fix: ExplainerCardsPortal relies on CopilotKit internal DOM structure

[GeneralJerel]

Description

Severity: P2 — Fragility

ExplainerCardsPortal in apps/app/src/components/explainer-cards.tsx:108-118 reaches into CopilotKit's internal DOM via welcomeScreen.children[0] and .lastElementChild. There are no guards for structural changes.

Failure mode

Cards silently disappear or render in the wrong location after a CopilotKit upgrade.

Suggested fix

• Add defensive checks: verify mainContent has the expected role/testid before inserting
• Consider a comment with the CopilotKit version this was tested against
• Alternatively, use a CopilotKit-provided extension point if one exists

From self-review finding F05