feat: build the branded desktop MVP shell mockup

[ibourgeois]

Context

As an early Katra user, I need the downloadable desktop app to present a branded, coherent product shell so I can understand where Katra is headed even before the full functionality exists.

Acceptance Criteria

• The current developer-oriented landing page is replaced with a branded MVP shell mockup.
• The UI uses the brand guidance from docs/brand/README.md and the existing Katra logo asset.
• The shell reflects the agreed desktop MVP navigation and information architecture.
• The shell is intentionally mock-first: it may include non-functional placeholders or empty states where functionality does not exist yet.
• Runtime and database connection diagnostics are removed from the main user-facing content.
• The new shell is suitable for gathering feedback from early users without implying unfinished features are already fully functional.
• The implementation uses feature flags so mock or staged UI areas can be hidden later if needed.

Notes

This work should prioritize layout, hierarchy, tone, and product direction over backend completeness.

The goal is not to fake a complete product. The goal is to replace the current smoke-test page with a more honest and useful shell that can guide early feedback.

The design should continue to feel desktop-first and should preserve room for future local-first and remote-instance workflows.

Out of Scope

• making every presented feature functional in the same PR
• website or repo marketing changes unrelated to the app shell