[codex] Optimize mobile chat layout

[verkyyi]

Summary

• Tighten mobile hero and chat panel spacing so the chatbot gets more viewport height.
• Show only one suggestion chip on mobile and keep the input visible.
• Remove the footer from mobile viewport layouts.
• Add E2E coverage for mobile one-hint/no-overflow behavior.

Why

Mobile Safari on iPhone was still cramped after removing the avatar. The chat panel had a fixed message minimum height, multiple visible suggestion chips, and footer/hero chrome competing with the input area.

Validation

• npm test -- src/__tests__/ChatPanel.test.tsx src/__tests__/Hero.test.tsx src/__tests__/App.test.tsx
• npm run build
• npx playwright test e2e/portfolio-layout.spec.ts
• Manual mobile metrics at 390x844: one visible suggestion, no horizontal overflow, footer hidden, input visible at bottom.