feat: remove prompting from AI cover generation

Commit notes

- Remove custom prompt input field from UI - Remove refinement/refine functionality entirely - Remove embedPackName toggle - Remove context override toggles (pack name, description, project name) - Simplify to one-click generation based purely on pack metadata - Backend now only uses pack name, description, categories, moods, and tags - Frontend flow: Generate → Select → Apply (no prompting steps)

AI covers are now generated automatically from: - Pack name (thematic hints) - Pack tags (visual hints) - Sample categories (visual elements) - Sample mood tags (color palette)

Co-authored-by: armin.naimi <[email redacted]>