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/package.json

@@ -18,6 +18,7 @@
     "@codemirror/view": "^6.43.8",
     "@lezer/highlight": "^1.2.3",
     "codemirror": "^6.0.2",
+    "overlayscrollbars": "^2.16.0",
     "paskia": "file:../../paskia/paskia-js",
     "transliteration": "^2.6.1",
     "vue": "^3.5.26",