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/views/ArtistsView.vue

@@ -76,9 +76,11 @@ onMounted(async () => {
 }
 .fc-artists__grid {
   display: grid;
-  /* min(440px, 100%) so a card never exceeds the viewport — on phones the
-     min-track collapses to 100% (single column) instead of overflowing. */
-  grid-template-columns: repeat(auto-fill, minmax(min(440px, 100%), 1fr));
+  /* min(360px, 100%) so a card never exceeds the viewport (phones collapse to
+     one column). 360 keeps a LONE column under ~732px wide, so the 3/1 preview
+     strip stays ≲244px tall and always fills the card width (no dead space),
+     while giving 2+ columns on a normal desktop. */
+  grid-template-columns: repeat(auto-fill, minmax(min(360px, 100%), 1fr));
   gap: 12px;
 }
 .fc-artists__sentinel {