Add block-update latency tracking, raw share counters, and shares bar UI

Log ZMQ→mining.notify latency in ckpool (patch 0005), expose a raw
reject counter (patch 0006), and surface both in the dashboard:

- Block latency card shows avg/last ms, wasted work, and block count
- SharesBar component shows session + all-time accepted/rejected with
  a Demon Slayer flame-slash animation on new shares
- Miners card info moved to MinersTable section header
- Latency stats and share counts persist across restarts via kv store
- Removed misleading pool-wide share stats from per-worker/user pages
  (ckpool doesn't expose per-user raw counts)

ui/src/lib/PoolOverview.svelte

@@ -11,8 +11,6 @@
   } from "../format";
 
   const data = $derived(snap.data!);
-  const miners = $derived(data.clients?.length ?? 0);
-  const workerCount = $derived(data.workers?.length ?? 0);
   const height = $derived(data.chain?.blocks ?? 0);
 
   const poolShare = $derived.by(() => {
@@ -56,6 +54,12 @@
 
   const totalHashes = $derived(data.cumulative_shares * 2 ** 32);
 
+  // Block update latency (ZMQ → mining.notify)
+  const latencyAvg = $derived(data.latency_avg_ms ?? 0);
+  const latencyLast = $derived(data.latency_last_ms ?? 0);
+  const latencyCount = $derived(data.latency_count ?? 0);
+  const staleWork = $derived(data.stale_work_hashes ?? 0);
+
   // Luck: best_share / cumulative_work * 100%.
   // 100% = exactly expected, >100% = lucky, <100% = unlucky.
   const luckPct = $derived.by(() => {
@@ -109,10 +113,22 @@
     </div>
   </div>
 
-  <div class="card">
-    <div class="stat-label">Miners</div>
-    <div class="stat-value">{miners}</div>
-    <div class="stat-sub">{workerCount} workers · {data.users?.length ?? 0} users</div>
+  <div
+    class="card"
+    title="Time from receiving a new block via ZMQ to pushing mining.notify to all miners. Wasted work = hashes spent mining the old block during this window."
+  >
+    <div class="stat-label">Block latency</div>
+    {#if latencyCount > 0}
+      <div class="stat-value">{latencyAvg}<span class="unit">ms</span></div>
+      <div class="stat-sub">
+        last {latencyLast}ms ·
+        <span class="wasted">{formatWork(staleWork)} wasted</span>
+        <span class="stat-dim">({latencyCount} blocks)</span>
+      </div>
+    {:else}
+      <div class="stat-value">—</div>
+      <div class="stat-sub">waiting for the next block</div>
+    {/if}
   </div>
 
   <div class="card">
@@ -171,6 +187,7 @@
       <span>{retarget.remaining} blocks · ~{retarget.eta}</span>
     </div>
   </div>
+
 </section>
 
 <style>
@@ -209,6 +226,13 @@
     font-weight: 400;
     margin-left: 0.15em;
   }
+  .stat-dim {
+    color: var(--fg-dim);
+    font-size: 0.9em;
+  }
+  .wasted {
+    color: var(--bad);
+  }
   .adj.up {
     color: var(--good);
   }