add a top progress bar during slow navigations #2

fixes TAN-1062 (tangled issue 4635, back buttons feel janky). on back and forward the url changes at once but the page only swaps after its data loads, so nothing shows that the navigation is happening.

• NavigationProgress shows a thin bar at the top of the viewport while a navigation is pending, after a 150ms delay so fast navigations show nothing
• the bar is mounted in the root layout, with reduced motion falling back to a static bar
• second commit uses a neutral colour (bg-foreground-placeholder) instead of the info colour. the exact gray is not decided yet

first commit, before and after (the bar is the line at the top of the after half):

second commit, light and dark:

pnpm test, eslint, pnpm check and prettier clean on changed files.