Epic B: Landing and app shell baseline

[mjkatgithub]

Goal

Introduce a real landing page and restore app shell layout foundations for
future customizable layouts.

Scope

• / route becomes dedicated landing page
• Landing includes explicit login CTA and native download placeholder
• NuxtLayout reactivation as public/app shell baseline

Definition of Done

• All sub-issues are created and linked to this epic
• Each sub-issue has acceptance criteria and test checklist
• Public and app layout boundaries are clearly prepared