Replace best share subtitle with luck indicator

Shows luck as best_share / total_work * 100%. Green for >100% (lucky),
red for <100% (unlucky), multiplier format for >=1000%. Hover tooltip
explains the metric.

ui/src/lib/PoolOverview.svelte

@@ -56,6 +56,20 @@
 
   const totalHashes = $derived(data.cumulative_shares * 2 ** 32);
 
+  // Luck: best_share / cumulative_work * 100%.
+  // 100% = exactly expected, >100% = lucky, <100% = unlucky.
+  const luckPct = $derived.by(() => {
+    const best = data.best_diff;
+    const work = data.cumulative_shares;
+    if (!best || !work || work <= 0) return 0;
+    return (best / work) * 100;
+  });
+  const luckLabel = $derived.by(() => {
+    if (!data.best_diff || !data.cumulative_shares) return "—";
+    if (luckPct >= 1000) return `${(luckPct / 100).toFixed(1)}x`;
+    return luckPct.toFixed(0) + "%";
+  });
+
   // Block-height flash on network-wide new block.
   let prevHeight = $state(0);
   let heightFlash = $state(false);
@@ -84,7 +98,15 @@
   <div class="card">
     <div class="stat-label">Best share</div>
     <div class="stat-value">{formatDifficulty(data.best_diff)}</div>
-    <div class="stat-sub">all-time pool record</div>
+    <div
+      class="luck-sub"
+      class:lucky={luckPct > 100}
+      class:unlucky={luckPct > 0 && luckPct < 100}
+      title="Luck = best share / total work. 100% means your best share matches the work done. Above 100% is lucky (found a harder share than expected), below is unlucky."
+    >
+      <span class="luck-value">{luckLabel}</span>
+      <span class="luck-label">luck</span>
+    </div>
   </div>
 
   <div class="card">
@@ -194,6 +216,35 @@
     color: var(--bad);
   }
 
+  .luck-sub {
+    display: flex;
+    align-items: baseline;
+    gap: 0.4em;
+    margin-top: 0.45em;
+    cursor: help;
+  }
+  .luck-value {
+    font-size: 1.15rem;
+    font-weight: 700;
+    font-variant-numeric: tabular-nums;
+    letter-spacing: -0.01em;
+  }
+  .luck-label {
+    color: var(--fg-dim);
+    font-size: 0.85rem;
+    font-weight: 400;
+    text-transform: uppercase;
+    letter-spacing: 0.04em;
+  }
+  .luck-sub.lucky .luck-value {
+    color: var(--good);
+    text-shadow: 0 0 10px rgba(92, 224, 168, 0.35);
+  }
+  .luck-sub.unlucky .luck-value {
+    color: var(--bad);
+    text-shadow: 0 0 10px rgba(255, 107, 107, 0.25);
+  }
+
   .height-card {
     transition: box-shadow 0.3s, border-color 0.3s;
   }