UI branding and customization (loading page, nav chrome, theming)

[nbenn]

Summary

Deferred from v3. Three pieces:

• Customizable cold-start loading page — v2 ships a default blockyard-branded spinner. Make it configurable: custom HTML template, logo URL, messaging via a [ui] config section.
• In-app navigation chrome — navbar with app name, app switcher for navigating between deployed apps without returning to the dashboard.
• General branding — configurable logo, colors, landing page content for the app browser.

Not blocking any v3 work. Revisit after the operations and runtime tracks land.