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/components/realtime-controls.tsx

@@ -3,7 +3,7 @@ import { StyleSheet, useUnistyles } from "react-native-unistyles";
 import { MicOff, Square } from "lucide-react-native";
 import { VolumeMeter } from "./volume-meter";
 import { useRealtime } from "@/contexts/realtime-context";
-import { FOOTER_HEIGHT } from "@/contexts/footer-controls-context";
+import { FOOTER_HEIGHT } from "@/constants/layout";
 
 const CONTROL_BUTTON_SIZE = 48;
 const VERTICAL_PADDING = (FOOTER_HEIGHT - CONTROL_BUTTON_SIZE) / 2;