Fix UnionYears in user-state-manager to utilize yet unspecified retirement date

- [ ] move the default value for retireDate from user-state-context to this function (?) so that it can see when the retire date is yet unset. Right now, it is impossible to know whether it is the starting value. This may require other changes to support the new default value of retire date.
- [ ] Use some of the code in the comments from user-state-manager.tsx line 94 and also inline comments to utilize whether retireDate had previously been set.

Start by reading user-state-manager.tsx, especially line 94 and its inline comments, then compare how retireDate is initialized in user-state-context. Trace how UnionYears determines whether a retirement date was previously set; done means an unset date remains distinguishable from the starting value and the related default-value changes are supported.

Written by the indexing model from the issue text.