feat: expanded smart folder category coverage (Gaming, RideSharing, FoodDelivery, SeasonalSales, Wellness, Shopping)

- Added 6 new EmailCategory enum values
- HeuristicClassifier rewritten with domain and subject-keyword rules:
  - RideSharing: Uber, Lyft, Bolt, Free Now, Grab etc.
  - FoodDelivery: DoorDash, Uber Eats, Deliveroo, Just Eat, Grubhub etc.
  - Gaming: Steam, Epic, EA, Xbox, PlayStation, Nintendo, Blizzard, Riot etc.
  - Shopping: Amazon, eBay, Etsy, ASOS, Zalando, Shein etc.
  - SeasonalSales: subject keywords (sale, % off, promo, black friday etc.)
  - Wellness: Strava, Garmin, Peloton, MyFitnessPal, Headspace, Nike etc. + subject hints
  - Priority ordering prevents ride-share/food receipts misclassifying as Finance
- SidebarCounts maps all new slugs to their EmailCategory
- frontend folderToRequest() and CAT_SLUG map all new categories
- Existing emails will pick up new categories on next sync

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/InboxIntel.Domain/Enums/Enums.cs

@@ -10,7 +10,13 @@ public enum EmailCategory
     Spam = 4,
     Promotional = 5,
     Social = 6,
-    Notification = 7
+    Notification = 7,
+    Gaming = 8,
+    RideSharing = 9,
+    FoodDelivery = 10,
+    SeasonalSales = 11,
+    Wellness = 12,
+    Shopping = 13
 }
 
 /// <summary>State machine for a sync run.</summary>