Fix nav_path_input write-after-widget error on invalid path

Use _sync_nav_path flag to defer the revert to the next rerun, before
the widget is instantiated.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -63,7 +63,7 @@ with st.sidebar:
             st.session_state.loaded_file = None
             st.rerun()
         else:
-            st.session_state.nav_path_input = str(st.session_state.current_dir)
+            st.session_state._sync_nav_path = True
             st.rerun()
 
     # --- Favorites System ---