refactor: implement footer controls context for smooth transitions

Codex implemented a portal/context architecture to eliminate layout jank:

- Created FooterControlsProvider context with register/unregister methods
- GlobalFooter now reserves fixed height (88px + insets) for all modes
- Both AgentInputArea and RealtimeControls stack absolutely in same space
- Single shared Reanimated value drives cross-fade (250ms)
- Agent screen registers controls via context instead of rendering inline
- Content padding adjusted to account for footer height
- Borders and safe area moved to shared container
- AgentInputArea simplified to pure component (no internal animations)

This guarantees:
- No layout shifts during transitions
- Smooth cross-fade between control modes
- No gaps or double borders
- Both controls never occupy layout space simultaneously

Implementation by OpenAI Codex per architectural recommendation.

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

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

packages/app/src/contexts/footer-controls-context.tsx

@@ -0,0 +1,49 @@
+import { createContext, useCallback, useContext, useMemo, useState } from "react";
+import type { ReactNode } from "react";
+
+export const FOOTER_HEIGHT = 88;
+
+interface FooterControlsContextValue {
+  controls: ReactNode | null;
+  registerFooterControls: (controls: ReactNode) => void;
+  unregisterFooterControls: () => void;
+}
+
+const FooterControlsContext = createContext<FooterControlsContextValue | undefined>(undefined);
+
+export function FooterControlsProvider({ children }: { children: ReactNode }) {
+  const [controls, setControls] = useState<ReactNode | null>(null);
+
+  const registerFooterControls = useCallback((nextControls: ReactNode) => {
+    setControls(nextControls);
+  }, []);
+
+  const unregisterFooterControls = useCallback(() => {
+    setControls(null);
+  }, []);
+
+  const value = useMemo(
+    () => ({
+      controls,
+      registerFooterControls,
+      unregisterFooterControls,
+    }),
+    [controls, registerFooterControls, unregisterFooterControls],
+  );
+
+  return (
+    <FooterControlsContext.Provider value={value}>
+      {children}
+    </FooterControlsContext.Provider>
+  );
+}
+
+export function useFooterControls() {
+  const context = useContext(FooterControlsContext);
+
+  if (!context) {
+    throw new Error("useFooterControls must be used within a FooterControlsProvider");
+  }
+
+  return context;
+}