fix(offline): gate library listing to downloaded-only when offline (#10)

The connectivity store now drives the "downloaded only" view so an
offline library page shows just on-device media, with the server catalog
revealed only when "Show all server media" is toggled.

TRACES: UR-052 | DR-078, DR-079

src/lib/services/offlineCatalog.ts

@@ -11,7 +11,7 @@
 // It also owns the `showServerCatalog` UI flag (the offline banner toggle that
 // reveals greyed-out, non-downloaded server media).
 //
-// TRACES: UR-002
+// TRACES: UR-002, UR-052 | DR-078
 
 import { writable, type Writable } from "svelte/store";
 import { commands } from "$lib/api/bindings";