ux(failing-sources): zebra striping + hover highlight on rows

Operator-flagged 2026-06-01: with the failing-sources panel expanded,
all rows have the same flat low-opacity background, no visual track
from the artist name on the left to the Logs/Retry buttons on the
right. Hard to tell which row a button belongs to when scanning down
a list of 17.

Three pure-CSS changes (no DOM, no logic):
- 3px gap between rows (was 2px) — slightly more breathing room
- Even rows get a darker surface tint (zebra striping)
- Hover paints the whole row in a low-opacity error tint, so moving
  the cursor toward Logs/Retry lights up the whole horizontal band
  and the eye traces back to the artist name automatically

frontend/src/components/subscriptions/FailingSourcesCard.vue

@@ -90,13 +90,24 @@ async function onViewLogs(s) {
 .fc-fail__title { font-weight: 600; }
 .fc-fail__body {
   padding: 0 14px 10px;
-  display: flex; flex-direction: column; gap: 2px;
+  display: flex; flex-direction: column; gap: 3px;
 }
 .fc-fail__row {
   display: flex; align-items: center; gap: 10px;
   padding: 6px 8px;
   border-radius: 4px;
   background: rgb(var(--v-theme-surface) / 0.4);
+  /* Zebra striping + hover highlight: gives the eye a horizontal track
+   * from the artist name on the left to the Logs/Retry buttons on the
+   * right. Operator-flagged 2026-06-01 — without these, all rows blur
+   * together and it's hard to pick which row a button belongs to. */
+  transition: background 80ms ease;
+}
+.fc-fail__row:nth-child(even) {
+  background: rgb(var(--v-theme-surface) / 0.55);
+}
+.fc-fail__row:hover {
+  background: rgb(var(--v-theme-error) / 0.18);
 }
 .fc-fail__artist { font-weight: 600; white-space: nowrap; }
 .fc-fail__count { flex: 0 0 auto; }