The previous fix put both the touch-target min-height and the
active-state border-bottom on the same <a>: centering the text
within a 2.75em box pushed the border ~0.7em away from it, a real
visible regression argus caught by actually rendering the CSS rather
than reasoning about the box model abstractly.
Split the two concerns onto two elements: the <a> (.shell-nav-link)
owns the full-height tappable box, an inner <span>
(.shell-nav-link-text) wrapping just the label owns the underline,
so the indicator stays directly under the text regardless of the
tappable box's height.
Verified: tsc clean, build succeeds, screenshotted close-up (600x100)
and at 320px — underline sits flush under the text in both, no gap.
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