feat(ui): add floating variant to PageBack for sticky nav

- Add `floating` prop to pin PageBack to viewport on lg+ screens
- Switch styling from glass-subtle to glass with shadow for better contrast
- Use floating variant on SettingsLayout

web/src/components/PageBack.test.tsx

@@ -52,7 +52,7 @@ describe("PageBack", () => {
     expect(mocks.navigate).toHaveBeenCalledWith(-1);
   });
 
-  it("applies the documented positioning and glass-subtle styling", () => {
+  it("applies the documented positioning and glass styling", () => {
     render(
       <MemoryRouter>
         <PageBack />
@@ -61,13 +61,24 @@ describe("PageBack", () => {
 
     const button = screen.getByRole("button", { name: "Go back" });
     expect(button).toHaveClass(
-      "glass-subtle",
+      "glass",
       "absolute",
       "top-4",
-      "left-4",
+      "left-2",
       "z-20",
       "rounded-full",
       "p-1.5",
     );
   });
+
+  it("pins to the viewport on lg+ when floating is set", () => {
+    render(
+      <MemoryRouter>
+        <PageBack floating />
+      </MemoryRouter>,
+    );
+
+    const button = screen.getByRole("button", { name: "Go back" });
+    expect(button).toHaveClass("lg:fixed", "lg:left-[268px]");
+  });
 });

web/src/components/PageBack.tsx

@@ -3,16 +3,25 @@ import { useNavigate } from "react-router";
 
 interface PageBackProps {
   label?: string;
+  /**
+   * When true, pins the button to the viewport on lg+ so it stays visible
+   * while scrolling. The offset matches the app sidebar (260px) so the
+   * button sits just inside the page content area.
+   */
+  floating?: boolean;
 }
 
-export default function PageBack({ label = "Go back" }: PageBackProps) {
+export default function PageBack({ label = "Go back", floating = false }: PageBackProps) {
   const navigate = useNavigate();
+  const position = floating
+    ? "absolute top-4 left-2 sm:top-6 lg:fixed lg:left-[268px]"
+    : "absolute top-4 left-2 sm:top-6";
   return (
     <button
       type="button"
       aria-label={label}
       onClick={() => navigate(-1)}
-      className="glass-subtle text-muted-foreground hover:text-foreground absolute top-4 left-4 z-20 flex items-center justify-center rounded-full p-1.5 transition-colors sm:top-6 sm:left-6"
+      className={`glass text-foreground hover:bg-accent ${position} z-20 flex items-center justify-center rounded-full p-1.5 shadow-md transition-colors`}
     >
       <ChevronLeft className="size-5" />
     </button>

web/src/pages/SettingsLayout.tsx

@@ -156,8 +156,8 @@ export default function SettingsLayout() {
   return (
     <div className="min-h-[100dvh]">
       <main className="page-shell-wide relative flex min-h-[100dvh] flex-col py-4 sm:py-6">
-        <PageBack />
-        <div className="page-header gap-5">
+        <PageBack floating />
+        <div className="page-header mt-10 gap-5 sm:mt-12">
           <div className="min-w-0 space-y-3">
             <h1 className="page-title text-[clamp(2rem,4vw,3rem)]">Settings</h1>
             <p className="page-subtitle text-sm sm:text-base">