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>

frontend/src/components/widgets.jsx

@@ -109,6 +109,12 @@ const CAT_SLUG = {
   Social:       '/app/folder/social',
   Notification: '/app/folder/automated',
   Promotional:  '/app/folder/shopping',
+  Shopping:     '/app/folder/shopping',
+  Gaming:       '/app/folder/gaming',
+  RideSharing:  '/app/folder/ridesharing',
+  FoodDelivery: '/app/folder/food',
+  SeasonalSales:'/app/folder/sales',
+  Wellness:     '/app/folder/wellness',
   Spam:         '/app/folder/spam',
   Newsletter:   '/app/search?q=newsletter',
   Personal:     '/app/search?q=is:unread',