feat(masthead): compact navbar mode for non-Library pages

[oto-macenauer-absa]

Feature Description

The masthead should render in two distinct modes depending on the current page:

• Library (landing): full-height hero with eyebrow badge, large title, subtitle, and sub-nav
• App pages: compact navbar (~42px) showing only the sub-nav (Library + current app crumb) and a brand wordmark outside the constrained inner container on the left

Problem / Opportunity

The full-height masthead wastes vertical space on documentation app pages where the sub-app already supplies its own heading. A compact navbar gives users immediate access to navigation without scrolling past a large hero.

Acceptance Criteria

1. Library page renders the full masthead (unchanged)
2. All other pages render the compact masthead at ~42px height
3. Eyebrow, title paragraph, and subtitle are hidden on app pages
4. "Knowledge base" brand wordmark appears left of the nav at ≥768px, outside the mp-masthead-inner constraint
5. Brand hides below 768px; nav takes full width
6. mp-masthead--compact class drives all compact styles via CSS; no JS required

Proposed Solution

• Add mp-masthead--compact class to .mp-masthead when activeApp exists
• Render mp-masthead-brand as a direct sibling before .mp-masthead-inner (not inside it) so it escapes the max-width: 72rem centering
• CSS: compact masthead is display: flex; align-items: center; brand is flex-shrink: 0 with padding-left; inner is flex: 1

Dependencies / Related

Affects src/components/Masthead.astro and src/styles/marketplace.css