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/sync/service.go

@@ -203,6 +203,14 @@ func (s *Service) IncrementalSync(ctx context.Context) error {
 	return s.db.FinishSyncRun(ctx, runID, n, nil)
 }
 
+// ResetAll deletes every synced activity (and its laps/samples/kind
+// assignments) and rewinds the backfill watermark, so the next Backfill
+// call performs a genuinely fresh pull from Garmin instead of resuming from
+// wherever the previous one left off. Workout kinds are left untouched.
+func (s *Service) ResetAll(ctx context.Context) error {
+	return s.db.ResetAllSyncedData(ctx)
+}
+
 func (s *Service) fetchAndStoreWindow(ctx context.Context, startDate, endDate string) (int, error) {
 	activities, err := s.garmin.GetActivities(ctx, startDate, endDate, 500)
 	if err != nil {