fix(web): reserve scrollbar gutter to prevent layout shift

When page content grows past the viewport, the vertical scrollbar
appearing used to shrink the layout width and nudge the left sidebar.
Reserve stable gutter space (with overflow-y: scroll fallback) so the
main layout stays put during loading and navigation.

web/src/assets/css/main.less

@@ -3,6 +3,14 @@
     --content-main: 620px;
 }
 
+// Reserve vertical scrollbar space so content doesn't jump horizontally
+// when pages go from short (no bar) to tall (bar appears).
+html {
+    scrollbar-gutter: stable;
+    // Fallback for browsers without scrollbar-gutter support
+    overflow-y: scroll;
+}
+
 .app-container {
     margin: 0;