simplified code across the app and consolidated duplicated helpers

lib/widget/prosemirror/pm_rich_text.dart

@@ -152,22 +152,15 @@ class _PmRichTextState extends State<PmRichText> {
 
   double? _fontSize(dynamic raw, double base) {
     if (raw is! String) return null;
-    final match = RegExp(r'^(\d+(?:\.\d+)?)(px|rem|em|%)$').firstMatch(raw);
-    if (match == null) return null;
-    final value = double.parse(match.group(1)!);
-    final double size;
-    switch (match.group(2)) {
-      case 'px':
-        size = value;
-      case 'rem':
-        size = value * 16;
-      case 'em':
-        size = base * value;
-      case '%':
-        size = base * value / 100;
-      default:
-        return null;
-    }
+    final length = parsePmCssLength(raw);
+    if (length == null) return null;
+    final value = length.value;
+    final size = switch (length.unit) {
+      'px' => value,
+      'rem' => value * 16,
+      'em' => base * value,
+      _ => base * value / 100,
+    };
     return size.clamp(8, 72);
   }