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/lib/components/library/MediaCard.svelte

@@ -1,11 +1,13 @@
-<!-- TRACES: UR-037, UR-051, UR-052, UR-068 | DR-042, DR-068, DR-078, DR-119 -->
+<!-- TRACES: UR-037, UR-051, UR-052, UR-068 | DR-042, DR-068, DR-078, DR-119, DR-292 -->
 <script lang="ts">
   import type { MediaItem, Library } from "$lib/api/types";
   import { truncateMiddle } from "$lib/utils/truncateMiddle";
   import { downloads } from "$lib/stores/downloads";
   import { isConnected } from "$lib/stores/connectivity";
   import { showServerCatalog } from "$lib/services/offlineCatalog";
-  import { auth } from "$lib/stores/auth";
+  import { deviceContentIds } from "$lib/services/downloadedCatalog";
+  import { isServerOnly as computeIsServerOnly } from "$lib/utils/serverOnly";
+  import { queueOfflineDownload } from "$lib/services/queueOfflineDownload";
   import CachedImage from "$lib/components/common/CachedImage.svelte";
   import FavoriteButton from "$lib/components/FavoriteButton.svelte";
   import { favoriteOverrides, resolveIsFavorite } from "$lib/stores/favorites";
@@ -143,11 +145,21 @@
   const isQueued = $derived(downloadInfo?.status === "pending");
   // Actively transferring (as opposed to merely queued/pending for reconnect).
   const isActivelyDownloading = $derived(downloadInfo?.status === "downloading");
-  // "Server only" = offline, reveal on, and not already downloaded or actively
-  // transferring. A `pending` (queued-for-reconnect) item stays server-only so
-  // it can show the Queued badge in place of the queue button.
+  // "Server only" = offline, reveal on, and nothing on the device behind this
+  // card — neither its own download nor, for a container, its children's (the
+  // album case: only tracks carry download rows). A `pending`
+  // (queued-for-reconnect) item stays server-only so it can show the Queued
+  // badge in place of the queue button. Shared with the list view; see
+  // $lib/utils/serverOnly.
   const isServerOnly = $derived(
-    isMediaItem && !$isConnected && $showServerCatalog && !isDownloaded && !isActivelyDownloading,
+    computeIsServerOnly({
+      isMediaItem,
+      isConnected: $isConnected,
+      revealServerCatalog: $showServerCatalog,
+      isDownloaded,
+      isActivelyDownloading,
+      hasDeviceContent: $deviceContentIds.has(item.id),
+    }),
   );
 
   // The heart is about an item, so libraries never get one, and a greyed
@@ -165,29 +177,13 @@
   async function queueForDownload(e: Event) {
     e.stopPropagation();
     if (!isMediaItem) return;
-    const media = item as MediaItem;
-    const userId = auth.getUserId();
-    if (!userId) {
-      queueError = "Not signed in";
-      return;
-    }
     try {
       queueError = null;
-      // Derive a sensible on-disk path; the backend heals stream_url on reconnect.
-      const filePath = `downloads/${media.id}`;
-      await downloads.downloadItem(
-        media.id,
-        userId,
-        filePath,
-        undefined,
-        undefined,
-        media.name,
-        media.artists?.join(", ") ?? undefined,
-        media.albumName ?? undefined,
-      );
+      await queueOfflineDownload(item as MediaItem);
     } catch (err) {
       log.error("Failed to queue download:", err);
-      queueError = "Failed to queue";
+      queueError =
+        err instanceof Error && err.message === "Not signed in" ? err.message : "Failed to queue";
     }
   }