✨ Auth views: optional brand accent slot above the title

[PierreBrisorgueil]

signin/signup views are fully stack-owned; a downstream consumer cannot add a small brand accent (logo variant, illustration) above the card title without forking the view file and breaking ISO.

Proposal: an optional config-driven component hook (e.g. config.sign.accentComponent) rendered centered above the title in signin/signup when set. Never rendered on lockout/reset/verify surfaces. Default behavior unchanged.

Acceptance: downstream injects the accent purely via config + its own project module.

[PierreBrisorgueil]

Simplest acceptable form: a configurable image — config.sign.accentImage (path/URL rendered centered above the signin/signup title when set). Component hook optional/follow-up. Still never rendered on lockout/reset/verify.