Implement CollectionController for managing card collection UI, including search, filter, sort, paginImplements all tasks in #50:
- Infinite scroll card grid using CardPrefab instances with object pooling
- Async Scryfall card image loading via ImageCache.GetSprite()
- Search bar wired to GET /api/collection?search=...
- Filter sidebar: color, type, set, rarity, owned/missing dropdowns
- Sort dropdown: name, CMC, color, type, price
- Card detail panel with flip animation trigger and full card view
- "Add to Deck" button routes to DeckBuilder with card stored in PlayerPrefs
- Pagination / load-more button with page info label
- Mobile: filter drawer auto-hides on narrow screens, collapsible via toggle buttonation, and card detail display.