Keep every site-shell nav link visible on phones #44

Below 560px, public/site-shell.css:283-289 makes .null-shell-nav scroll sideways and hides its scrollbar (scrollbar-width: none, ::-webkit-scrollbar { display: none }). The AI tools toggle sits right after it.

Measured on /script.html in Chrome:

• 375px wide: the nav is 253px wide but its content is 312px. "Control" runs from x=286 to 328 and sits half under the toggle at x=316. "Renders" (x=331-373) is fully off-screen.
• 320px wide: only 198px of 312px are visible.

Nothing shows that the bar scrolls, so on a phone the Renders page looks like it doesn't exist.

Fix direction: show all seven links plus the toggle from 320px to 560px. Options: smaller icon targets in that range, a wrap to a second row, or an edge fade with scroll-snap so overflow is obvious. Keep tap targets at 44px under pointer: coarse (lines 296-306). Also keep the current page's link in view, for example with scrollIntoView({ inline: "nearest" }) on mount in site-shell.js.

Done when at 320, 375, 414 and 560px every nav link is reachable and visible or clearly scrollable, nothing overlaps the AI toggle, and the page has no horizontal scroll.