convert container from Messages-only app to regular iOS app,
move shared code (models, api client, store) to Shared/ group,
add app group entitlements for cross-process data sharing,
rewrite StickerStore for shared UserDefaults + container,
create SwiftUI app entry point, simplify extension to read-only browser
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
