As an instruktør I want a display name shown next to shared sessions so I know who trained what

[ChristopherRotnes]

Summary

When joint-class history (#138) is visible, each session row should show the instruktør's name — not just their email. This requires a `display_name` field on the user profile and a safe way to expose it to co-instruktører at the same gym without leaking unrelated personal data.

Priority

Low (blocked by #138 — shared sessions must be visible before names matter)

Data model

-- Extend existing profiles table (or create one if it doesn't exist)
alter table profiles add column display_name text;

RLS: `display_name` should be readable by anyone at the same gym (same `sporty_business_unit_id` via `user_gyms`); only the owner can update it.

UI spec (Carbon g100)

• Settings → Konto section: Carbon `TextInput` for display name (max 50 chars), save button
• Joint-class history rows (As an instruktør I want to see joint session history for a class slot so I can avoid repeating what a colleague already trained #138): show `display_name` (or email fallback) as a secondary label

Acceptance criteria

• `display_name` column added to `profiles` (or equivalent) via Supabase migration
• RLS: same-gym users can read each other's `display_name`; only owner can write
• Settings form to set/update display name
• Joint-class history view uses `display_name` with email fallback
• Verified by developer on dev/qa/prod before closing

Out of scope

• Profile photos / avatars
• Display name visible outside the same gym

[ChristopherRotnes]

Delivered in PR #146 — profiles.display_name column, Settings Konto TextInput, same-gym RLS, display name shown in class history panel and own exercise list.