Scheduling, background art, and create fixes

SlideDrift's LinkedIn scheduling flow now feels closer to composing the real post.

The scheduler adds a calendar view, list filters for upcoming posts and drafts, a feed-style composer, timezone-aware time picking, caption assist, draft saving, and publish-now controls where publishing is available. Scheduled and published rows keep the rendered carousel PDF pinned to the post.

The editor also adds a Background Art option for slide and global backgrounds. You can choose curated vector patterns, search and filter the gallery, switch between Brand Kit colors and the preset palette, and adjust fit and intensity.

Create is a little smoother too: recent carousel shortcuts wait for guest-claim checks before loading, and generation defaults now preserve more of the selected format, story mode, and template preferences.