refactor: simplify footer architecture by removing global footer pattern

Replace portal/registry pattern with direct rendering where each screen
owns its footer. This eliminates FooterControlsContext indirection and
makes the code follow standard React composition patterns.

- Remove GlobalFooter and FooterControlsProvider from _layout.tsx
- Add HomeFooter component rendered directly in home screen
- Render AgentInputArea directly in agent screen (not via registration)
- Add keyboard animation and safe area handling to AgentInputArea
- Move FOOTER_HEIGHT constant to shared constants/layout.ts
- Delete footer-controls-context.tsx and global-footer.tsx

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

packages/app/src/constants/layout.ts

@@ -0,0 +1 @@
+export const FOOTER_HEIGHT = 75;