fix: keep dashboard shell dimensions stable

surfaces/shell/src/browser/HostApp.tsx

@@ -115,8 +115,10 @@ const shellStyles = {
   actions: { display: "flex", gap: designToken("spacing.sm"), justifyContent: "center" },
   aside: {
     borderRight: `${designToken("border.width")} solid ${designToken("border.divider")}`,
+    boxSizing: "border-box",
     minWidth: "13rem",
     padding: designToken("spacing.lg"),
+    width: "13rem",
   },
   groupLabel: {
     color: designToken("text.muted"),
@@ -140,10 +142,17 @@ const shellStyles = {
     color: designToken("text.primary"),
     display: "grid",
     fontFamily: designToken("typography.family-sans"),
-    gridTemplateColumns: "auto 1fr",
+    gridTemplateColumns: "13rem minmax(0, 1fr)",
     minHeight: "100vh",
+    minWidth: 0,
+    width: "100%",
+  },
+  main: {
+    boxSizing: "border-box",
+    minWidth: 0,
+    overflowX: "auto",
+    padding: designToken("spacing.lg"),
   },
-  main: { padding: designToken("spacing.lg") },
   message: {
     color: designToken("text.secondary"),
     display: "grid",
@@ -531,7 +540,7 @@ export function HostApp(props: HostAppProps): ReactElement {
   const { groups, ungrouped } = bootstrap.navigation;
 
   return (
-    <div style={shellStyles.layout}>
+    <div data-testid="host-shell" style={shellStyles.layout}>
       <header style={shellStyles.header}>
         <h1 style={shellStyles.heading}>Ekklesia</h1>
         <Button data-testid="host-logout" onClick={() => void signOut()}>