fix(profile): one display-name bound, checked after trimming

The setup flow capped a display name at 50 characters while the profile
took 80, and the profile checked its minimum before trimming, so a name
of only spaces was stored as an empty string. Every surface now reads
one bound (80), the profile trims before validating, and only spaces
means no name on both paths: the profile clears it to null, the setup
flow leaves it unset.

docs/api/openapi.yaml

@@ -26651,9 +26651,10 @@ components:
                 - OTHER
             - type: "null"
         displayName:
+          description: Trimmed before its bound is checked. Null, an empty string or only spaces clears it; an omitted field is
+            left untouched.
           anyOf:
             - type: string
-              minLength: 1
               maxLength: 80
             - type: "null"
         locale:
@@ -33760,8 +33761,7 @@ components:
           maxLength: 64
         displayName:
           type: string
-          minLength: 1
-          maxLength: 50
+          maxLength: 80
         heightCm:
           type: number
           minimum: 50