ui: reserve the scrollbar gutter so the top bar stops shifting between pages

Pages tall enough to scroll showed a ~15px scrollbar; short pages didn't — so the
centered top bar jumped left/right as you navigated. scrollbar-gutter: stable on html
(SPA app.css + the server-rendered share pages) keeps the layout width constant. No-op
on overlay-scrollbar platforms (mobile), which never shifted.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/app.css

@@ -34,7 +34,10 @@
 }
 
 * { box-sizing: border-box; }
-html { -webkit-text-size-adjust: 100%; scroll-behavior: smooth; }
+/* Always reserve the scrollbar gutter so the centered top bar / content doesn't jump
+   left-right between pages that scroll and pages that don't. (No-op on overlay-scrollbar
+   platforms like mobile, which never shifted anyway.) */
+html { -webkit-text-size-adjust: 100%; scroll-behavior: smooth; scrollbar-gutter: stable; }
 body {
   margin: 0;
   background: var(--bg);