Replace per-kind reclassify with a single global reclassify that respects manual and Race locks

Manual assignments are the user's definitive word and Race assignments come
from a hard Garmin fact (eventType.typeKey), not a retunable rule -- neither
is ever touched by reclassify again, and Race can no longer be set by hand
via the review queue. Adds an Activities page so this locked/unlocked status
is visible per workout, since Review Queue only ever showed unresolved items.

backend/internal/api/server.go

@@ -67,9 +67,10 @@ func (s *Server) Router() http.Handler {
 			r.Get("/", s.handleListWorkoutKinds)
 			r.Get("/{id}", s.handleGetWorkoutKind)
 			r.Put("/{id}", s.handleUpdateWorkoutKind)
-			r.Post("/{id}/reclassify", s.handleReclassifyKind)
 		})
 
+		r.Post("/reclassify", s.handleReclassifyAll)
+
 		r.Route("/review-queue", func(r chi.Router) {
 			r.Get("/", s.handleReviewQueue)
 			r.Post("/{activityID}/resolve", s.handleResolveReview)