Scope block reorg detection to the current chain

Blocks are now stamped with the Bitcoin network name ("main", "test",
"signet") at ingest time. The reconcile loop's Pass 2 skips any stored
block whose chain differs from the node's current chain, preventing
testnet blocks from being falsely orphaned after switching back to
mainnet. Legacy rows with an empty chain field fall through unchanged.

The UI shows a blue "test" / "signet" badge next to blocks from a
non-current network so operators can distinguish cross-chain history
from genuine reorg-orphaned blocks.

ui/src/lib/BlocksTable.svelte

@@ -10,6 +10,8 @@
   const explorerBase = $derived(
     explorerBaseFor(snap.data?.chain?.chain, snap.data?.mempool_base_url),
   );
+
+  const currentChain = $derived(snap.data?.chain?.chain ?? "");
 </script>
 
 <section class="card">
@@ -36,6 +38,9 @@
                 {#if b.orphaned_at}
                   <span class="orphan-tag" title="Reorged out of the canonical chain at {b.orphaned_at}">orphaned</span>
                 {/if}
+                {#if b.chain && currentChain && b.chain !== currentChain}
+                  <span class="chain-tag" title="Mined on {b.chain} — current node is {currentChain}">{b.chain}</span>
+                {/if}
               </td>
               <td class="hash-cell">
                 {#if b.hash}
@@ -121,6 +126,18 @@
     text-decoration: none;
     vertical-align: middle;
   }
+  .chain-tag {
+    display: inline-block;
+    margin-left: 0.4em;
+    padding: 0.05em 0.4em;
+    border-radius: 3px;
+    background: rgba(80, 160, 220, 0.15);
+    color: rgb(100, 170, 220);
+    font-size: 0.7em;
+    font-weight: 600;
+    text-decoration: none;
+    vertical-align: middle;
+  }
   .unit {
     color: var(--fg-dim);
     font-size: 0.75em;