ui: disable trackpad two-finger swipe-to-navigate

Adds 'overscroll-behavior-x: none' on html / body so the browser
treats horizontal trackpad swipes as no-ops instead of back / forward
navigation. Vertical scroll is unaffected. Prevents the classic
'I lost my work because I scrolled the canvas' design-tool gotcha.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/index.html

@@ -53,6 +53,15 @@
       color: var(--ink);
       font-family: 'VT323', 'Courier New', monospace;
       letter-spacing: 0.05em;
+      /* Stop the trackpad two-finger horizontal swipe from triggering
+         the browser's back / forward navigation — which would throw
+         away in-progress design work. `overscroll-behavior-x: none`
+         tells the browser the page does not want to "rubber-band"
+         horizontally, and Chrome / Edge / Firefox honor it as a
+         swipe-nav disable. Vertical scroll is unaffected. Safari
+         respects it on newer versions; older Safari users still get
+         the gesture but most users are on modern browsers. */
+      overscroll-behavior-x: none;
     }
 
     /* Thin scrollbars throughout — about half the default thickness. */