refactor(modal): replace bottom sheet with native modal and custom animations

Replace @gorhom/bottom-sheet with React Native's native Modal component and implement custom slide-in/slide-out animations using Reanimated. This provides better control over modal behavior and animations while reducing dependency on external libraries.

Key changes:
- Replace BottomSheetModal with native Modal component
- Implement custom slide animations using Reanimated shared values
- Add backdrop fade in/out animations
- Extract modal sections into focused components (ModalHeader, WorkingDirectorySection, AssistantSelector, ModeSelector, WorktreeSection)
- Add responsive layout support with compact/wide screen detection
- Fix keyboard animation to use transform instead of padding
- Improve modal lifecycle management with mount/unmount state tracking

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

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

packages/app/src/components/global-footer.tsx

@@ -110,7 +110,10 @@ export function GlobalFooter() {
             </Pressable>
 
             <Pressable
-              onPress={() => setShowCreateModal(true)}
+              onPress={() => {
+                console.log("[GlobalFooter] New Agent button pressed");
+                setShowCreateModal(true);
+              }}
               style={({ pressed }) => [
                 styles.footerButton,
                 pressed && styles.buttonPressed,