simplified code across the app and consolidated duplicated helpers

lib/widget/prosemirror/pm_node.dart

@@ -325,3 +325,12 @@ class PmUnknown extends PmNode {
 
   const PmUnknown({required this.rawType, this.children = const []});
 }
+
+final RegExp _cssLength = RegExp(r'^(\d+(?:\.\d+)?)(px|%|em|rem)$');
+
+/// Splits a CSS length like `12px` / `1.5em` / `50%` into value and unit.
+({double value, String unit})? parsePmCssLength(String raw) {
+  final match = _cssLength.firstMatch(raw);
+  if (match == null) return null;
+  return (value: double.parse(match.group(1)!), unit: match.group(2)!);
+}