fix(offline): one server-only rule for both library views

Two defects with one cause: "server only" was a private $derived inside
MediaCard.

The list view (what LibraryGrid renders when the stored view preference
is list) had no notion of it at all, so a library browsed as a list
offline showed every revealed item as an ordinary tappable row that plays
nothing, with no way to queue it.

And the rule asked the downloads store whether *this item id* was
downloaded — but only a playable leaf (Audio, Movie, Episode) ever has a
download row. An album's tracks carry them, the album does not, so a
fully downloaded album greyed itself out and offered to queue what was
already on the device.

The rule moves to the pure $lib/utils/serverOnly and both views call it.
The container half is answered by the backend rather than guessed at:
get_download_disk_usage().sizes already carries container subtotals
beside leaf sizes (DR-085), so deviceContentIds is membership in a
Rust-computed map, not a frontend list of which item types are
containers. That map was loaded only by the Downloads page, so the shell
primes it at startup and re-reads it whenever the offline gate settles.
Queueing is shared too, since the list view had no copy to diverge from.

TRACES: UR-052, UR-055 | DR-292 | UT-257, UT-258

src/routes/+layout.svelte

@@ -19,6 +19,8 @@
     showServerCatalog,
     lastCatalogSync,
   } from "$lib/services/offlineCatalog";
+  import { downloadedCatalog } from "$lib/services/downloadedCatalog";
+  import { useOfflineFilterReload } from "$lib/composables/useOfflineFilterReload";
   import { playbackMode } from "$lib/stores/playbackMode";
   import { sessions } from "$lib/stores/sessions";
   import ReauthModal from "$lib/components/auth/ReauthModal.svelte";
@@ -109,6 +111,11 @@
     shellReservesBottomInset({ pathname, isAuthenticated: $isAuthenticated }),
   );
 
+  // The device-content map decides which revealed cards are greyed, so it has
+  // to be current whenever the offline gate settles — going offline, or
+  // flipping "Show all server media" — not just at startup. (DR-292)
+  useOfflineFilterReload(() => downloadedCatalog.refresh());
+
   // The offline banner is shell chrome like the rest, so it stays off the
   // full-screen player — where on the native video path it would paint a
   // stripe straight through the film (DR-291).
@@ -176,6 +183,14 @@
     const userId = get(auth).user?.id;
     if (userId) {
       downloads.refresh(userId).catch((err) => log.error("Initial downloads refresh failed:", err));
+      // Which containers hold downloaded children is a backend question
+      // (`get_download_disk_usage`), and the offline browse needs the answer to
+      // avoid greying out a fully downloaded album — its *tracks* carry the
+      // download rows, never the album. Primed here because the map used to be
+      // loaded only by the Downloads page (DR-292).
+      downloadedCatalog
+        .refresh()
+        .catch((err) => log.error("Initial downloaded-catalog refresh failed:", err));
     }
 
     // Start sync service for offline mutation queue