Overlay scrollbars via OverlayScrollbars

overflow: overlay is dead in current Chromium, so an appearing native
scrollbar shifted the layout again. OverlayScrollbars replaces it with
floating auto-hiding scrollbars that never reserve space (native window
scroll APIs unaffected on a body target). Themed via --os-* variables in
pagerite.css; the html scrollbar-color styling stays as no-JS fallback.
(package-lock.json is gitignored.)

frontend/src/assets/pagerite.css

@@ -67,31 +67,22 @@
 
 html {
   scroll-behavior: smooth;
-  /* Overlay scrollbars where supported (Chromium/WebKit, deprecated but
-     working): a scrollbar that appears covers a strip of the layout instead
-     of shifting it, and no space is reserved while absent — so 100vw always
-     equals the body width and .wide breakout margins stay exact. Ignored by
-     Firefox (unknown value), which keeps its thin classic scrollbar. */
-  overflow-y: overlay;
-  /* Lightweight scrollbar styling: thin, theme-muted thumb on a transparent
-     track where supported (Firefox: the two properties; WebKit: the rules
-     below). Full restyling deliberately not attempted. */
+  /* Native-scrollbar fallback styling (JS off or before pagerite.js runs):
+     thin, theme-muted thumb on a transparent track. With JS the scrollbars
+     are replaced by OverlayScrollbars (see pagerite.js) — floating,
+     auto-hidden scrollbars styled by the --os-* variables below, so they
+     never reserve layout space or shift the page when appearing. */
   scrollbar-width: thin;
   scrollbar-color: color-mix(in srgb, var(--muted) 45%, transparent) transparent;
 }
 
-::-webkit-scrollbar {
-  width: 0.5rem;
-  height: 0.5rem;
-}
-
-::-webkit-scrollbar-thumb {
-  background: color-mix(in srgb, var(--muted) 45%, transparent);
-  border-radius: 0.25rem;
-}
-
-::-webkit-scrollbar-track {
-  background: transparent;
+.os-scrollbar {
+  --os-size: 0.5rem;
+  --os-thumb-bg: color-mix(in srgb, var(--muted) 45%, transparent);
+  --os-thumb-hover-bg: color-mix(in srgb, var(--muted) 65%, transparent);
+  --os-thumb-active-bg: var(--muted);
+  --os-track-bg: transparent;
+  --os-thumb-border-radius: 0.25rem;
 }
 
 body {