Fix state-mutating getter in LengthItem.Length

Remove dangerous side effect where the Length property getter
was modifying the LengthInputValue field. Property getters should
be pure and not cause state mutations, as this violates the principle
of least surprise and can cause issues with data binding, serialization,
and debugging.

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Co-Authored-By: Claude <noreply@anthropic.com>

CutList/Models/LengthItem.cs

@@ -22,7 +22,6 @@ namespace CutList.Models
 
                     if (double.TryParse(input, out d))
                     {
-                        LengthInputValue += "\"";
                         return d;
                     }