refactor: reuse FullingBrand in landing header

[Che-Zhu]

Summary

• reuse the shared FullingBrand component in the landing header
• make the landing wordmark typography (22px size, 24px line height, and -0.5px tracking) canonical across landing, login, and dashboard headers
• remove the duplicated landing brand markup and obsolete responsive styles
• add regression coverage for canonical rendering, accessibility, and surface-specific destinations

Why

The landing page duplicated the shared brand lockup introduced for login and dashboard. That component-boundary omission allowed the wordmark typography and responsive behavior to drift between surfaces. Centralizing the lockup keeps brand dimensions and accessible labeling consistent.

Impact

Landing, login, and dashboard now render the same brand treatment. Dashboard retains its /workspace destination; landing and login continue to link to /.

Validation

• npm test — 40 tests passed
• npm run lint
• npm run build
• git diff --check

Fixes #188