fix(artists): card preview dead-space + empty-state flicker on first load

Two operator-reported Artists-view bugs.

Layout: ArtistCard previews used aspect-ratio: 3/1 + max-height: 220px — when a
lone grid column got wide (>~660px), the max-height made the aspect-ratio box
shrink its own WIDTH to keep the ratio, leaving dead space beside the 3
thumbnails. Dropped max-height (kept the min-height floor) so the strip fills the
card width; lowered the grid min 440→360px so a lone column stays <732px (strip
≲244px) and desktop gets 2+ columns.

Flicker: isEmpty checked !loading, but on the first render loading is still false
(the initial loadMore fires in onMounted, after paint) so "No artists match"
flashed for a frame before the spinner/data. Added a reactive `loaded` flag (true
after the first load attempt, reset on reset()); isEmpty now also requires it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/discovery/ArtistCard.vue

@@ -49,9 +49,11 @@ function onCardClick() {
   position: relative;
   display: grid; grid-template-columns: repeat(3, 1fr);
   gap: 2px; aspect-ratio: 3 / 1;
-  /* Explicit floor + ceiling so tall source images can't escape the
-     preview slot even on browsers where aspect-ratio doesn't compute. */
-  min-height: 150px; max-height: 220px;
+  /* Floor so tall source images can't escape the slot. NO max-height: an
+     aspect-ratio box capped by max-height shrinks its own WIDTH to keep the
+     ratio, leaving dead space beside the previews on a wide (single-column)
+     card. The grid below keeps columns ≲400px so the strip stays a sane height. */
+  min-height: 120px;
   overflow: hidden;
   background: rgb(var(--v-theme-surface-light));
 }