The whole Cosmos UI exists, but it "lives" on one URL — you can't share a link to a specific picture, and the browser Back button does nothing. We'll fix that with a router by hand — no framework.

Since this is a single-page app (SPA), there's no real navigation between HTML files. Instead we use the URL hash (the part after #):

#/            → home
#/gallery     → gallery
#/collection  → collection
#/apod/2026-07-11 → one picture (shareable, bookmarkable)

The router is just a table of "name → function" and one event:

onRoute('gallery', renderGallery);
onRoute('detail', renderDetail);
window.addEventListener('hashchange', handleRoute);

The brilliance is in the hashchange event. It fires both when you click a link with a # and when you press the browser's Back/Forward. So the Back button works for free — we do nothing extra. That's the key insight.

Three capabilities we'll get:

• Deep links. #/apod/2026-07-11 opened fresh (with no data) must fetch that one picture itself.
• URL-synced search. #/gallery?q=moon — the search becomes a shareable link and survives a refresh.
• A working Back button. Browse the gallery → a detail → back, and the browser takes you back.

And two traps: how not to create a hundred history entries per search keystroke (replaceState), and why you never split ?q=moon&days=30 by hand (URLSearchParams).