fix(dashboard): welcome a new account instead of welcoming it back

The header always said "welcome back", including to an account the
setup flow had created a few minutes earlier.

/api/auth/me now publishes the account's createdAt, and the greeting
welcomes an account in its first day ("Welcome, name.") and welcomes
back every other one, including an account whose older server does not
send the field. Until the account is known, during the SSR pass and the
hydration render, the line is the time-of-day greeting alone, so
nothing claims "back" before the client knows it is true.

docs/api/openapi.yaml

@@ -86362,6 +86362,12 @@ components:
           type: string
         username:
           type: string
+        createdAt:
+          type: string
+          format: date-time
+          pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d:[0-5]\d(?:\.\d+)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
+          description: When the signed-in account was created. The web greets an account in its first day as new rather than
+            welcoming it back.
         timezone:
           type: string
           description: "The IANA zone the server cuts this account's days in: the stored zone, or the instance default when the
@@ -86437,6 +86443,7 @@ components:
       required:
         - id
         - username
+        - createdAt
         - timezone
         - glucoseUnit
         - features