fix(image-gen): silently ignore existing directory in ensureDirectoryExists on Android

Capacitor's Filesystem.mkdir() throws on Android when the directory already exists,
even with recursive: true. This caused image downloads via exportByUrl to fail on
second attempt since the chatbox_ai_exports directory was already created.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>