refactor: reuse FullingBrand across landing and login headers

[Che-Zhu]

Problem

The brand lockup in the top-left header is visually inconsistent between the landing page (/) and the login page (/login).

The two pages render the same logo asset at the same visible mark size (24px), but use different wordmark typography:

Surface	Implementation	Wordmark styles
/	Duplicated markup in app/(landing)/_components/landing-client.tsx + landing.module.css	22px / 24px, letter-spacing -0.5px
/login	Shared components/fulling-brand.tsx	20px (text-xl) / line-height 1, default letter-spacing

The dashboard header already uses the same shared FullingBrand component as the login page.

Both implementations were introduced together in #182, so this is a component-boundary omission rather than a compatibility requirement.

Why this should be shared

docs/design.md defines the Brand Logo Mark as the reusable top-left identity anchor and applies the design system to both public and authentication pages. Maintaining two copies allows brand dimensions and accessibility behavior to drift.

Proposed change

• Use FullingBrand in the landing header instead of duplicating its Link, Image, mark crop, and wordmark markup.
• Make the canonical logo/wordmark sizing, line-height, tracking, color, and accessible label live in the shared component.
• Preserve only layout-specific positioning in the landing page stylesheet.
• Remove the obsolete .brand, .brandMark, and .brandMark img rules after migration.

Acceptance criteria

• /, /login, and the dashboard render the shared FullingBrand component.
• The visible logo mark and “Fulling” wordmark have identical dimensions and typography across those headers.
• Header layout remains correct on desktop and compact/mobile breakpoints.
• The brand link still points to the intended destination and retains the accessible Fulling home label.
• A regression check covers the shared rendering or computed brand styles.