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/utils/serverOnly.ts

@@ -0,0 +1,52 @@
+/**
+ * Is a card "server only" — revealed by the offline "Show all server media"
+ * toggle, but with nothing on the device behind it?
+ *
+ * Such cards render greyed out, are inert to tap (there is nothing to play),
+ * and offer to queue a download for the next reconnect instead.
+ *
+ * This lives in its own module because two surfaces answer the question — the
+ * grid (`MediaCard`) and the list (`LibraryListView`) — and they disagreed:
+ * the list had no notion of server-only at all, so switching a library to list
+ * view offline turned every non-downloaded item back into a normal, tappable
+ * row that plays nothing.
+ *
+ * `hasDeviceContent` is the fix for the second half of that defect. An item's
+ * *own* download row only ever exists for a playable leaf (Audio, Movie,
+ * Episode); an album or a season never has one, so a fully downloaded album
+ * greyed itself out and offered to queue what was already on the device. The
+ * caller passes the backend's answer — `get_download_disk_usage().sizes`
+ * carries container subtotals as well as leaf sizes — rather than the frontend
+ * deciding which item types are containers, which is taxonomy that belongs in
+ * Rust.
+ *
+ * TRACES: UR-052 | DR-292 | UT-257
+ */
+
+export interface ServerOnlyInput {
+  /** False for a `Library` tile — a library is never queued or greyed. */
+  isMediaItem: boolean;
+  /** Server reachability (`$isConnected`). */
+  isConnected: boolean;
+  /** The offline banner's "Show all server media" toggle. */
+  revealServerCatalog: boolean;
+  /** This item's own download row is `completed`. */
+  isDownloaded: boolean;
+  /** This item's own download row is actively transferring. */
+  isActivelyDownloading: boolean;
+  /** The device holds bytes at or under this item (leaf file or container). */
+  hasDeviceContent: boolean;
+}
+
+export function isServerOnly({
+  isMediaItem,
+  isConnected,
+  revealServerCatalog,
+  isDownloaded,
+  isActivelyDownloading,
+  hasDeviceContent,
+}: ServerOnlyInput): boolean {
+  if (!isMediaItem) return false;
+  if (isConnected || !revealServerCatalog) return false;
+  return !isDownloaded && !isActivelyDownloading && !hasDeviceContent;
+}