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/stores/viewMode.test.ts

@@ -0,0 +1,69 @@
+/**
+ * Display preference (grid/list) is a second view onto the library `viewMode`
+ * store. The Settings Display section and the library page-header toggle both
+ * drive it via `library.setViewMode`, so verifying the store writes through and
+ * persists covers the shared data path for both controls.
+ *
+ * TRACES: UR-029 | DR-077 | UT-*
+ */
+
+import { describe, it, expect, beforeEach, beforeAll, afterAll, vi } from "vitest";
+import { get } from "svelte/store";
+
+const STORAGE_KEY = "jellytau-view-mode";
+
+// jsdom here doesn't expose localStorage; stand in a minimal implementation.
+const backing = new Map<string, string>();
+const localStorageShim = {
+  getItem: (key: string) => backing.get(key) ?? null,
+  setItem: (key: string, value: string) => void backing.set(key, value),
+  removeItem: (key: string) => void backing.delete(key),
+  clear: () => backing.clear(),
+};
+
+// The library store imports the auth store and tauri events at module load;
+// neither is exercised by these tests.
+vi.mock("@tauri-apps/api/event", () => ({
+  listen: vi.fn(async () => () => {}),
+}));
+vi.mock("./auth", () => ({ auth: { getUserId: vi.fn() } }));
+
+beforeAll(() => {
+  vi.stubGlobal("localStorage", localStorageShim);
+});
+
+afterAll(() => {
+  vi.unstubAllGlobals();
+});
+
+describe("viewMode display preference", () => {
+  beforeEach(() => {
+    backing.clear();
+    vi.resetModules();
+  });
+
+  it("setViewMode writes through to the derived store", async () => {
+    const { library, viewMode } = await import("./library");
+    library.setViewMode("list");
+    expect(get(viewMode)).toBe("list");
+    library.setViewMode("grid");
+    expect(get(viewMode)).toBe("grid");
+  });
+
+  it("setViewMode persists the choice to localStorage", async () => {
+    const { library } = await import("./library");
+    library.setViewMode("list");
+    expect(backing.get(STORAGE_KEY)).toBe("list");
+  });
+
+  it("a persisted list preference is restored on load", async () => {
+    backing.set(STORAGE_KEY, "list");
+    const { viewMode } = await import("./library");
+    expect(get(viewMode)).toBe("list");
+  });
+
+  it("defaults to grid when nothing is persisted", async () => {
+    const { viewMode } = await import("./library");
+    expect(get(viewMode)).toBe("grid");
+  });
+});