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/stores/connectivity.ts

@@ -7,7 +7,7 @@
 // mirrors status; it does not decide reachability itself. navigator.onLine is
 // advisory and triggers a recheck rather than forcing offline.
 // See docs/architecture/07-connectivity.md.
-// TRACES: UR-002 | DR-013
+// TRACES: UR-002, UR-043, UR-052 | DR-013, DR-055, DR-079
 
 import { writable, derived } from "svelte/store";
 import { browser } from "$app/environment";
@@ -231,8 +231,20 @@ export const connectivity = createConnectivityStore();
 // Derived stores for convenience
 export const isOnline = derived(connectivity, ($c) => $c.isOnline);
 export const isServerReachable = derived(connectivity, ($c) => $c.isServerReachable);
+// "Connected" follows backend reachability ALONE — not navigator.onLine.
+// The Rust ConnectivityMonitor (fed by real repository traffic) is the source
+// of truth (DR-055); navigator.onLine is advisory and can be wrong (server
+// unreachable on a live device link — server down, wrong LAN, dropped VPN —
+// still reports online). Folding it in kept `isConnected` true in exactly those
+// cases and prevented the offline "downloaded only" gate from ever closing.
+// navigator.onLine stays a *trigger* for a recheck (the online/offline
+// listeners call checkServerReachable), never a *term* in this decision.
+// The startup default (isServerReachable: true) is intentionally optimistic —
+// a brief full-catalog flash before the first probe beats flipping the app to
+// "offline" on launch. See docs/architecture/07-connectivity.md.
+// TRACES: UR-052 | DR-079
 export const isConnected = derived(
   connectivity,
-  ($c) => $c.isOnline && $c.isServerReachable
+  ($c) => $c.isServerReachable
 );
 export const connectionError = derived(connectivity, ($c) => $c.connectionError);