fix(ui): prevent mobile viewport horizontal scroll

The mobile layout root had no horizontal-overflow guard, so any stray
descendant wider than the screen (a long token, an over-wide element)
made the entire viewport scroll sideways. Add overflow-x: clip to the
mobile root. Using clip rather than hidden leaves overflow-y computed as
visible, preserving native body scrolling and the sticky breadcrumb.

Add a Layout regression test asserting the mobile root carries
overflow-x-clip and the desktop root carries overflow-clip.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/components/Layout.test.tsx

@@ -723,4 +723,51 @@ describe("Layout", () => {
       root.unmount();
     });
   });
+
+  async function renderLayoutRoot(): Promise<{ root: ReturnType<typeof createRoot>; rootEl: HTMLElement }> {
+    const root = createRoot(container);
+    const queryClient = new QueryClient({
+      defaultOptions: { queries: { retry: false } },
+    });
+    await act(async () => {
+      root.render(
+        <QueryClientProvider client={queryClient}>
+          <Layout />
+        </QueryClientProvider>,
+      );
+    });
+    await flushReact();
+    const rootEl = container.firstElementChild as HTMLElement;
+    return { root, rootEl };
+  }
+
+  it("clips horizontal overflow on the mobile layout root so the viewport can't scroll sideways", async () => {
+    mockSidebarState.isMobile = true;
+    mockSidebarState.sidebarOpen = false;
+    const { root, rootEl } = await renderLayoutRoot();
+
+    expect(rootEl.tagName).toBe("DIV");
+    expect(rootEl.className).toContain("bg-background");
+    // The mobile root must clip horizontal overflow to prevent a stray wide
+    // descendant from making the whole viewport scroll sideways. clip (not
+    // hidden) keeps overflow-y visible so body scroll keeps working.
+    expect(rootEl.classList.contains("overflow-x-clip")).toBe(true);
+    expect(rootEl.classList.contains("overflow-hidden")).toBe(false);
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
+
+  it("clips overflow on the desktop layout root", async () => {
+    mockSidebarState.isMobile = false;
+    const { root, rootEl } = await renderLayoutRoot();
+
+    expect(rootEl.className).toContain("bg-background");
+    expect(rootEl.classList.contains("overflow-clip")).toBe(true);
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
 });

ui/src/components/Layout.tsx

@@ -469,7 +469,11 @@ export function Layout() {
       <div
       className={cn(
         "bg-background text-foreground pt-[env(safe-area-inset-top)]",
-        isMobile ? "min-h-dvh" : "flex h-dvh flex-col overflow-clip",
+        // overflow-x-clip on mobile keeps a stray wide descendant from making the
+        // whole viewport scroll horizontally. clip (not hidden) leaves overflow-y
+        // computed as visible, so native body scroll + the sticky breadcrumb keep
+        // working.
+        isMobile ? "min-h-dvh overflow-x-clip" : "flex h-dvh flex-col overflow-clip",
       )}
       >
       <a