Replaces the per-page headers with one persistent site header, so the main navigation is available on every page without going through the home page.

What's included

• Shared header (SiteHeader, PageHeader) rendered once in App, so it stays mounted while navigating. Only the title, subtitle and help text change per page. The current page is highlighted from the route.
• Uniform size: the banner has the same height on every page, and the title is one line with a fixed height. The map page gets the same banner, with the map below it.
• Smaller navigation buttons to keep the banner compact, with the same look as before.
• Sea gradient on the banner and a wave edge that hangs below it, so it blends with both the page background and the map (fixes the white line above the map).
• Removed the "Hovedside" button and the HomeLink component.
• Smoother page changes: each page starts at the top and fades in. Fetched data (places, challenges, user) is cached with useFetch, so returning to a page does not flash a loading state. The map page now uses fetchAllPlaces instead of its own fetch code.
• Sailing quote at the bottom of every page (PageFooter).
• Wheel design: simplified the ship's wheel (fewer details, no gaps between segments).
• Bug fix: effects no longer return the result of scrollTo / the spin callback, which made React crash with "destroy is not a function" in some browsers.

Notes

• Stacked on feat/challenges-page, so this PR targets that branch until the earlier PRs are merged.
• The map page is now taller than the viewport (header above the map), so you scroll a little to see all of it.