More reliable generation, exports, and brand defaults

Brand profiles are easier to reuse. Signed-in users can duplicate a saved brand profile, mark one custom profile as the default, clear that default later, and upload a logo for supported brand slots.

Generation now checks signed-in credit availability before starting and shows a clear error if a stream ends before SlideDrift receives the final completion event. That helps avoid treating partial placeholder slides as finished work.

Exports have stronger delivery safeguards. SlideDrift validates export requests more tightly, signs stored share downloads, and counts export allowance after a file or share link is delivered instead of before rendering.

The Create and carousel library surfaces also handle empty and failed-loading states more clearly, including retry paths for recent carousels and templates.