✨ Make the notFound view illustration configurable

[PierreBrisorgueil]

The stack home.notfound.view.vue renders title/message/buttonText from config.app.notFound, but a downstream consumer cannot add a visual (illustration/brand component) above the title without editing the stack file (breaking the ISO merge contract).

Proposal: support an optional, config-driven illustration hook — e.g. config.app.notFound.component (resolved via the existing dynamic-component pattern) or a named slot rendered above the title when provided. Default behavior unchanged.

Acceptance: a downstream project can inject its own 404 visual purely from project-owned files (config + project module).

[PierreBrisorgueil]

Simplest acceptable form: a configurable image — config.app.notFound.image (path/URL, rendered above the title when set, sized via optional imageWidth). The dynamic-component hook can be a follow-up; the image path alone unblocks downstream branding.