Add Crate Digger feature for swipe-based music discovery

- POST /api/recommendations/crate endpoint generates diverse crate of discoveries
- POST /api/recommendations/crate-save endpoint saves individual picks
- CrateDigger.tsx page with card UI, pass/save buttons, slide animations
- Progress bar, save counter, and end-of-crate stats summary
- Added to nav, routing, and API client

frontend/src/components/Layout.tsx

@@ -13,6 +13,7 @@ const baseNavItems = [
   { path: '/discover', label: 'Discover', icon: Compass },
   { path: '/analyze', label: 'Analyze', icon: Lightbulb },
   { path: '/generate-playlist', label: 'Create Playlist', icon: ListPlus },
+  { path: '/crate', label: 'Crate Dig', icon: Disc3 },
   { path: '/bandcamp', label: 'Bandcamp', icon: Store },
   { path: '/compatibility', label: 'Taste Match', icon: Users },
   { path: '/saved', label: 'Saved', icon: Heart },