Kill ckpool on bitcoind failure so miners can failover

When bitcoind is unreachable for 3+ consecutive polls and no block
submission is pending, SIGTERM the ckpool process so miners disconnect
and failover to backup pools. Adds a red dashboard banner when
Bitcoin Core is down.

ui/src/lib/HealthBanners.svelte

@@ -25,10 +25,22 @@
     // Alarm when ZMQ is 3+ min older than the last tip change.
     return zmqAge > tipAge + 180;
   });
+
+  const bitcoindDown = $derived(snap.data != null && !snap.data.bitcoin_ok);
 </script>
 
-{#if submitGap > 0 || zmqStale}
+{#if bitcoindDown || submitGap > 0 || zmqStale}
   <div class="banners">
+    {#if bitcoindDown}
+      <div class="banner error">
+        <span class="icon">!</span>
+        <div class="text">
+          <strong>Bitcoin Core is unreachable.</strong>
+          Stratum server has been stopped — miners will failover to backup pools.
+          Service will resume automatically when Bitcoin Core recovers.
+        </div>
+      </div>
+    {/if}
     {#if submitGap > 0}
       <div class="banner warn">
         <span class="icon">!</span>
@@ -75,6 +87,11 @@
     border-color: rgb(220, 170, 60);
     color: rgb(220, 180, 100);
   }
+  .banner.error {
+    background: rgb(50, 20, 20);
+    border-color: rgb(220, 60, 60);
+    color: rgb(230, 120, 120);
+  }
   .icon {
     flex: 0 0 auto;
     width: 1.5em;