feat(chrome): shared account menu and global app header

Move account actions (Settings, Downloads, Display preferences, Sign
out) out of the library-only header into a shared AccountMenu anchored in
a global AppHeader, available on every authenticated non-immersive
screen. Add a layoutShell helper deciding where chrome shows, expose
serverName/serverUrl auth stores, and a display view-mode preference. The
settings page also gains the UR-053 WiFi-only toggle.

TRACES: UR-054 | DR-075, DR-076, DR-077

src/lib/utils/layoutShell.test.ts

@@ -18,6 +18,7 @@ import { describe, it, expect } from "vitest";
 import {
   showBottomNav,
   showGlobalMiniPlayer,
+  showGlobalHeader,
   routeOwnsLayout,
   showBottomUi,
 } from "./layoutShell";
@@ -87,6 +88,33 @@ describe("routeOwnsLayout", () => {
   });
 });
 
+describe("showGlobalHeader", () => {
+  it("shows the root-owned header (with account menu) on home, search, downloads", () => {
+    // The whole point of UR-054: account actions reachable without going to Library.
+    expect(showGlobalHeader(authed("/"))).toBe(true);
+    expect(showGlobalHeader(authed("/search"))).toBe(true);
+    expect(showGlobalHeader(authed("/downloads"))).toBe(true);
+  });
+
+  it("does NOT show on library (it renders its own AppHeader)", () => {
+    expect(showGlobalHeader(authed("/library"))).toBe(false);
+    expect(showGlobalHeader(authed("/library/abc"))).toBe(false);
+  });
+
+  it("does NOT show on settings (owns its content; no account menu needed there)", () => {
+    expect(showGlobalHeader(authed("/settings"))).toBe(false);
+  });
+
+  it("does NOT show on the immersive player or login", () => {
+    expect(showGlobalHeader(authed("/player/x"))).toBe(false);
+    expect(showGlobalHeader(authed("/login"))).toBe(false);
+  });
+
+  it("hides when unauthenticated", () => {
+    expect(showGlobalHeader({ pathname: "/", isAuthenticated: false })).toBe(false);
+  });
+});
+
 describe("structural invariant: every route that shows bottom UI has a scroller above it", () => {
   // With the in-flow model, "the bottom UI is a flex sibling below a scroller"
   // must hold on every route where it shows. That scroller is provided by