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.ts

@@ -0,0 +1,33 @@
+import { goto } from "$app/navigation";
+
+/**
+ * Navigate "back" using real browser/Android history when possible, falling
+ * back to an explicit path otherwise.
+ *
+ * Hardcoded `goto(backPath)` always sends the user to a fixed screen, which
+ * loses track of where they actually came from (e.g. reaching the genres list
+ * from different entry points). Preferring `history.back()` keeps the back
+ * affordance consistent with the platform back gesture and the browser/Android
+ * hardware back button.
+ *
+ * We only use history when there is somewhere to go back to *within the app*.
+ * On a fresh deep-link (history length 1, or an external referrer) we fall back
+ * to `fallbackPath` so the user never gets stranded or bounced out of the app.
+ */
+export function navigateBack(fallbackPath: string): void {
+  if (canGoBack()) {
+    history.back();
+  } else {
+    goto(fallbackPath);
+  }
+}
+
+/**
+ * True when there is in-app history to pop. `history.length > 1` means the user
+ * navigated here from another page in this session rather than landing here
+ * directly (deep link, refresh, or first load).
+ */
+function canGoBack(): boolean {
+  if (typeof history === "undefined") return false;
+  return history.length > 1;
+}