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.

src/hooks/use-workouts.ts

@@ -290,6 +290,14 @@ export type WorkoutActivityInsight = {
 } | null;
 
 export interface WorkoutDetailPayload extends WorkoutListEntry {
+  /**
+   * The session's local calendar day, `YYYY-MM-DD`. Computed server-side
+   * from `startedAt` in the user's timezone — never re-derived here, so a
+   * browser in a different zone can't move the workout to another day. It
+   * addresses the day-scoped reads the detail page renders around the
+   * session.
+   */
+  dayKey: string;
   minHr: number | null;
   stepCount: number | null;
   elevationM: number | null;