[editor] Sections as collections of any tagged asset type

- Extend CollectionConfig with `collectionSource`: pages / images / text / video / all

- Collection rendering per source type: pages as page cards, images as masonry grid, text as list, video as video grid

- Extend `CollectionSettings.tsx` with source type selector

- Preview count updates live

- Add to collection shortcut from asset gallery (tag assignment)

**Files:** `src/components/salon/editor/CollectionSettings.tsx`, `src/types/blocks.ts`, `src/hooks/use-collection.ts`