fix(handover): show the link holder display names only

The claim preview is anonymous: whoever holds the link has proved
nothing yet. It listed each guardian's grant id and fell back to their
login name. It now carries the display name alone, null when there is
none, and the page says "a guardian" for that.

The notifications to the guardians fell back to the new owner's login
name when the record had no display name. They now use sentences about
the person the guardian looked after instead.

docs/api/openapi.yaml

@@ -41806,14 +41806,15 @@ components:
           items:
             type: object
             properties:
-              grantId:
-                type: string
               displayName:
-                type: string
+                anyOf:
+                  - type: string
+                  - type: "null"
+                description: "The guardian's display name; null when they set none. Never a login name or an id: the holder of the link
+                  has proved nothing yet."
               proposal:
                 $ref: "#/components/schemas/ProfileClaimGuardianAccess"
             required:
-              - grantId
               - displayName
               - proposal
             additionalProperties: false