Merge backfill into Sync now, replace Full backfill with a destructive Reset all

Sync now now runs Backfill (resumes from the watermark, so a widened history
horizon is picked up automatically) then IncrementalSync then detail-fill, in
one click. "Full backfill" is gone -- in its place, "Reset all" (styled as a
destructive action, gated by a confirm dialog) wipes every synced activity
and its laps/kind assignments and rewinds the backfill watermark, so the next
sync performs a genuinely fresh pull instead of trying to patch up existing
rows with newer schema fields.

backend/internal/api/server.go

@@ -53,7 +53,7 @@ func (s *Server) Router() http.Handler {
 
 		r.Route("/sync", func(r chi.Router) {
 			r.Post("/run", s.handleSyncRun)
-			r.Post("/backfill", s.handleSyncBackfill)
+			r.Post("/reset", s.handleSyncReset)
 			r.Get("/runs", s.handleSyncRuns)
 			r.Get("/status", s.handleSyncStatus)
 		})