chore(discovery-v2): per-section item count 6 → 18

Reverts the diagnostic-only drop to 6 (used to verify whether item
density was the vertical-scroll bottleneck — it wasn't; the wheel
event handling was the root cause). Restores the user-facing default
of 18 items per section, which gives ~3 pages at FullHD and ~2 at 4K
with the current 220px card size.

cc

src/lib/discovery-v2/DiscoveryView.svelte

@@ -157,9 +157,9 @@
     // Each is independent so they race without blocking.
     const genreIds = Array.from(getSelectedGenreIds('home'));
     const [watch, index, resolved] = await Promise.all([
-      fetchReleaseWatch(20),
+      fetchReleaseWatch(18),
-      fetchDiscoverIndex(20, genreIds),
+      fetchDiscoverIndex(18, genreIds),
-      fetchHomeResolved(20),
+      fetchHomeResolved(18),
     ]);
     releaseWatch = watch;
     newReleases = index.newReleases;