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/voice-note-recording-overlay.tsx

@@ -2,7 +2,7 @@ import { View, Text, Pressable, ActivityIndicator } from "react-native";
 import { StyleSheet, useUnistyles } from "react-native-unistyles";
 import { X, ArrowUp, RefreshCcw } from "lucide-react-native";
 import { VolumeMeter } from "./volume-meter";
-import { FOOTER_HEIGHT } from "@/contexts/footer-controls-context";
+import { FOOTER_HEIGHT } from "@/constants/layout";
 import type { DictationStatus } from "@/hooks/use-dictation";
 
 interface VoiceNoteRecordingOverlayProps {