fix(ui): keep room for the scrollbar so pages do not shift

scrollbar-gutter: stable on the root: a page that scrolls (e.g. a wide
Kanban board) and one that does not have the same width, so switching
tabs no longer moves the layout. Overlay scrollbars are unaffected.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/index.css

@@ -29,6 +29,13 @@
   -webkit-tap-highlight-color: transparent;
 }
 
+/* Room for the scrollbar is always kept: pages that scroll (e.g. a wide Kanban board) and pages that do
+   not keep the same width, so nothing shifts when switching tabs. Overlay scrollbars (phones) are not
+   affected. */
+html {
+  scrollbar-gutter: stable;
+}
+
 body {
   background-color: var(--bg-app);
   color: var(--text-main);