Centralize chart data directory name

lib/models/chart.dart

@@ -1,5 +1,9 @@
 import 'dart:convert';
 
+/// Directory (relative to the deck) where linked chart CSVs are kept, so the
+/// data files stay tidily in one place — separate from images/media.
+const String chartDataDirName = 'data';
+
 /// Supported chart kinds for a chart slide.
 enum ChartType { bar, line, pie }
 

lib/services/file_service.dart

@@ -222,7 +222,7 @@ class FileService {
     final spec = ChartSpec.parse(s.customMarkdown);
     final src = spec.source;
     if (src == null) return s;
-    final rel = addAsset(src, 'data');
+    final rel = addAsset(src, chartDataDirName);
     if (rel == null) {
       return s.copyWith(
         customMarkdown: spec.copyWith(clearSource: true).toBlock(),