Add Presents Puzzle interactive to Discrete Math > Uncertainty

Created a new interactive puzzle that explores probability and search strategies:
- Alice opens boxes in sequential order (1, 2, 3, ...)
- Bob opens odds first, then evens (1, 3, 5, ..., 2, 4, 6, ...)
- 26 presents randomly distributed in 100 boxes
- Interactive visualization with two 10×10 grids
- Single simulation with animated opening process
- Batch simulation of 100 rounds to analyze win rates
- Added to Uncertainty theme page (first interactive in this category)

src/components/InteractiveIndex.tsx

@@ -270,6 +270,15 @@ const allInteractives: Interactive[] = [
     path: '/themes/discrete-math/structures/cube-coloring',
     theme: 'Discrete Math',
     dateAdded: '2025-01-22'
+  },
+  {
+    id: 'presents-puzzle',
+    title: 'The Presents Puzzle',
+    description: 'Explore a probability puzzle about search strategies. Charlie puts 26 presents in 100 boxes. Alice opens them in order while Bob opens odds first, then evens. Who is more likely to see all 26 presents first?',
+    tags: ['probability', 'search', 'simulation', 'expected-value', 'strategy', 'uncertainty'],
+    path: '/themes/discrete-math/uncertainty/presents-puzzle',
+    theme: 'Discrete Math',
+    dateAdded: '2025-11-19'
   }
 ];