Date a workout by the user's timezone on the detail endpoint

The detail payload carried the session's start timestamp but nothing that
said which local day it belongs to. Anything that wants the day around a
workout had to slice the ISO string, which is the browser's day, not the
person's: a 20:00 UTC session reads as the 15th in London and the 16th in
Auckland.

Resolve the day server-side from the user's timezone and ship it as
dayKey. Additive, so a native client that does not read it is untouched.

docs/api/openapi.yaml

@@ -19968,6 +19968,8 @@ components:
           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)))$
+        dayKey:
+          type: string
         durationSec:
           type: integer
           minimum: 0
@@ -20076,6 +20078,7 @@ components:
         - sportType
         - startedAt
         - endedAt
+        - dayKey
         - durationSec
         - distanceM
         - activeEnergyKcal