More reliable private template generation

Generating from a saved or selected template now keeps more of the template's authored visual system through the full generation flow.

When a template defines its own typography, sizing, colors, or rich text styling, SlideDrift avoids replacing those choices with a broad theme pass at the end of generation. Private saved templates are also checked against the signed-in owner during generation, so reusable designs you saved can be loaded more reliably when creating a fresh carousel.