fix: top nav height varying by route (flex shrink lock)

[jstortoise]

Top nav: keep a fixed height across routes

The header sits as the first child in AppLayout’s column flex stack (height: 100vh, overflow: hidden) above <main> with flexGrow: 1. Flex items default to flex-shrink: 1, so when heavier routes mount, the main area can exert shrink pressure on the header and the bar’s rendered height drifts slightly by route.

Change: Treat the header as a fixed-size flex sibling, not a shrinkable one:

• flexShrink: 0 and flexGrow: 0 so <main> cannot compress or stretch the bar.
• minHeight / maxHeight aligned with the existing height breakpoints (xs: 52, md: 56) so the bar stays one locked size.
• boxSizing: 'border-box' so the bottom border counts inside that height and totals stay consistent.

Active vs inactive nav links already both use a 1px bottom border (color only differs), so no link-level style changes were required.

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