feat(library): genre sliders, artist links, and navigation utils

- music landing: diverse per-genre album sliders (online counts /
  offline wide-probe fallback) and home-screen library shortcuts
- add ArtistLinks component and shared navigation/genreDiversity utils
- player/playback-mode refinements across Rust and frontend

src/lib/utils/navigation.test.ts

@@ -0,0 +1,35 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { navigateBack } from "./navigation";
+
+const goto = vi.fn();
+vi.mock("$app/navigation", () => ({
+  goto: (...args: unknown[]) => goto(...args),
+}));
+
+describe("navigateBack", () => {
+  beforeEach(() => {
+    goto.mockClear();
+  });
+
+  it("pops real history when there is in-app history to go back to", () => {
+    const back = vi.fn();
+    vi.spyOn(history, "back").mockImplementation(back);
+    vi.spyOn(history, "length", "get").mockReturnValue(3);
+
+    navigateBack("/library");
+
+    expect(back).toHaveBeenCalledOnce();
+    expect(goto).not.toHaveBeenCalled();
+  });
+
+  it("falls back to the given path on a fresh deep-link (no history)", () => {
+    const back = vi.fn();
+    vi.spyOn(history, "back").mockImplementation(back);
+    vi.spyOn(history, "length", "get").mockReturnValue(1);
+
+    navigateBack("/library/music");
+
+    expect(goto).toHaveBeenCalledWith("/library/music");
+    expect(back).not.toHaveBeenCalled();
+  });
+});