Most data routes run their load with ssr = false, so SvelteKit holds the
previous page on screen while the next one's data resolves — with no
feedback. NavProgress, mounted once in the root layout and driven by the
$navigating store, shows a thin top bar that trickles while a navigation is
pending and fades out on completion. Degrades to a static visible bar under
prefers-reduced-motion. New --z-nav-progress token sits above the fixed
header but below modals/toasts.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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