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>

goodnews/share.py

@@ -60,6 +60,8 @@ def _back_link_html(label: str = "Back") -> str:
 # Ported verbatim from HubBar.svelte's <style> (+ HubShell's .back), scoped to the same
 # class names so the bar looks pixel-identical to the SPA regardless of page palette.
 _TOP_BAR_CSS = """
+  /* reserve the scrollbar gutter so the bar doesn't jump left-right between pages */
+  html { scrollbar-gutter: stable; }
   @font-face { font-family:'Hanken Grotesk'; src:url('/fonts/hanken-var.woff2') format('woff2'); font-weight:400 700; font-style:normal; font-display:swap; }
   header.bar { display:flex; align-items:center; justify-content:space-between; max-width:1180px; width:100%; margin:0 auto; box-sizing:border-box; padding:26px clamp(18px,5vw,44px) 0; font-family:'Hanken Grotesk',ui-sans-serif,system-ui,sans-serif; background:none; border:none; }
   header.bar .brand { display:block; line-height:0; }