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.test.ts

@@ -0,0 +1,45 @@
+/**
+ * Tests for the "server only" card rule shared by the grid and the list view.
+ *
+ * TRACES: UR-052 | DR-292 | UT-257
+ */
+
+import { describe, it, expect } from "vitest";
+import { isServerOnly, type ServerOnlyInput } from "./serverOnly";
+
+const offlineReveal: ServerOnlyInput = {
+  isMediaItem: true,
+  isConnected: false,
+  revealServerCatalog: true,
+  isDownloaded: false,
+  isActivelyDownloading: false,
+  hasDeviceContent: false,
+};
+
+describe("isServerOnly", () => {
+  it("is true only offline, with the reveal on, for an item with nothing on the device", () => {
+    expect(isServerOnly(offlineReveal)).toBe(true);
+    expect(isServerOnly({ ...offlineReveal, isConnected: true })).toBe(false);
+    expect(isServerOnly({ ...offlineReveal, revealServerCatalog: false })).toBe(false);
+  });
+
+  it("never greys a library tile: there is nothing to queue", () => {
+    expect(isServerOnly({ ...offlineReveal, isMediaItem: false })).toBe(false);
+  });
+
+  it("does not grey the item's own completed or in-flight download", () => {
+    expect(isServerOnly({ ...offlineReveal, isDownloaded: true })).toBe(false);
+    expect(isServerOnly({ ...offlineReveal, isActivelyDownloading: true })).toBe(false);
+  });
+
+  it("does not grey a container whose children are on the device", () => {
+    // The regression: an album has no download row of its own — its *tracks*
+    // do — so a fully downloaded album greyed itself out and offered to queue
+    // what was already there.
+    expect(isServerOnly({ ...offlineReveal, hasDeviceContent: true })).toBe(false);
+  });
+
+  it("still greys a container with nothing downloaded under it", () => {
+    expect(isServerOnly({ ...offlineReveal, hasDeviceContent: false })).toBe(true);
+  });
+});