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

[LizAinslie]

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

[LizAinslie]

i18n: #60 (en template key for this name).