Signup: auto-create default calendar ({name}'s Calendar), admin-configurable

## Goal
On signup, automatically create a default calendar for the new user. Behavior remains **admin-configurable**.

## Copy lock (en)
One message template — **not** string concat:

`{name}'s Calendar`

Possessives don’t concat cleanly once humans translate; keep the whole string in one `en` message.

## Admin
- Toggle: enable/disable auto-create on signup
- Optional: override name template (still one message / config string, not concat)
- Document defaults in DEVELOPING / admin UI

## Acceptance
- [ ] New user gets one default calendar named from the template with their display/preferred name
- [ ] Admin can disable auto-create
- [ ] Admin can change the template string
- [ ] Uses i18n `en` message key (see sibling i18n ticket) — no hardcoded concat in Kotlin/UI