Fix selectedValues type error in MembersMultiSelect.tsx

Remove explicit ProjectMemberRow type annotation and use more
explicit type guard to ensure selectedValues is string[].

Fixes TypeScript error TS2345 where the map callback type was
incompatible with Editable<...> array items.

Refs #1007

Co-authored-by: karilint <19990676+karilint@users.noreply.github.com>

frontend/src/components/Project/MembersMultiSelect.tsx

@@ -185,8 +185,8 @@ const MembersMultiSelectDetail = () => {
       fieldName="now_proj_people"
       idFieldName="initials"
       selectedValues={editData.now_proj_people
-        .map((member: ProjectMemberRow) => member.initials)
-        .filter((initial): initial is string => !!initial)}
+        .map(member => member.initials)
+        .filter((initial): initial is string => typeof initial === 'string' && initial.length > 0)}
       editingAction={person =>
         setEditData({
           ...editData,