Custom template thumbnail upload

Admin users can now upload a custom thumbnail image for any Template or Preset, giving a more accurate visual representation than the system's auto-generated version. This is especially useful for open, wireframe-style templates where the default thumbnail doesn't clearly convey how the template is meant to be used. The option is available in the Template/Preset settings and can be accessed by anyone with "Modify" or "Create Templates" access. If no custom image is uploaded, the auto-generated thumbnail will continue to be used by default.