Show follow-up status after mobile composer expansion

[dillonzq]

Human comments

What was wrong

In #3601, the compact follow-up composer container collapsed its status footer with an unconditional descendant selector. On iOS, React correctly marked a focused composer with data-follow-up-composer-expanded and rendered the environment and context controls, but the container rule still forced that footer to zero height and opacity.

What changed

Scope the existing footer-collapse rule to follow-up composers without data-follow-up-composer-expanded, matching the neighboring compact composer rules. Add a CSS regression assertion that requires the expanded-state guard and rejects the former broad selector.

How you verified

• pnpm exec turbo run test --filter=@bb/app -- --run src/app.css.test.ts src/components/promptbox/FollowUpPromptBox.test.tsx (46 tests passed)
• pnpm exec turbo run build --filter=@bb/app
• git diff origin/main...HEAD --check

Fixes #3601

AGENT GENERATED

[Copilot]

🟢 Approval recommended

No unresolved blocking issues were identified.

Pull request overview

Fixes mobile follow-up composer footer visibility by scoping collapse styles to unexpanded composers.

Changes:

• Guards the footer-collapse selector with the expanded-state attribute.
• Adds a regression assertion for the corrected selector.

File summaries

File	Summary
apps/app/src/app.css.test.ts	Verifies the guarded selector.
apps/app/src/app.css	Preserves the footer when expanded.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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