feat: redesign landing page with SST design system

[Che-Zhu]

Summary

• redesign the Fulling landing page around SST's measured layout, typography, spacing, borders, and responsive behavior
• show the live GitHub star count through the cached server-side lookup
• establish app/globals.css as the single executable design-token source
• document the Fulling/SST visual system in docs/design.md
• migrate the current authentication, application shell, workspace overview, kubeconfig settings, dialogs, and error states away from the legacy UI system

Why

This is the first implementation phase of the product redesign tracked in #178. It gives Fulling a faithful SST-derived public landing page and a shared visual foundation for later workspace workflows.

Impact

• Rubik and IBM Plex Mono are now the only global type families
• legacy green, old neutral tokens, page-level design colors, filled primary CTAs, large radii, and routine shadows are removed
• shared controls use the same compact, flat, hairline-border system
• desktop and mobile landing layouts retain the approved SST composition

Validation

• corepack pnpm lint
• corepack pnpm test — 35 tests passed
• corepack pnpm build
• manual visual QA at 1363 × 964 and 390 × 844
• verified the landing preview CTA navigates to the login page without console errors

Closes #178