Add Chain column to blocks table and display human-readable chain names

Shows which network each block was mined on. Cross-network blocks
display a blue tag with dimmed row; same-network blocks show the
chain name in subdued text. Maps raw identifiers to readable names
(main → mainnet, testnet4 stays as-is).

ui/src/lib/BlocksTable.svelte

@@ -2,6 +2,13 @@
   import { snap } from "../stores/snapshot.svelte";
   import { formatAgo, formatDifficulty, explorerBaseFor } from "../format";
 
+  const chainDisplayName: Record<string, string> = {
+    main: "mainnet",
+  };
+  function displayChain(chain: string): string {
+    return chainDisplayName[chain] ?? chain;
+  }
+
   const blocks = $derived.by(() => {
     const b = snap.data?.recent_blocks ?? [];
     return [...b].reverse();
@@ -24,6 +31,7 @@
         <thead>
           <tr>
             <th>Height</th>
+            <th>Chain</th>
             <th>Hash</th>
             <th class="num">Reward</th>
             <th class="num">Winning share</th>
@@ -32,14 +40,20 @@
         </thead>
         <tbody>
           {#each blocks as b (b.height + "-" + b.found_at)}
-            <tr class:orphaned={!!b.orphaned_at}>
+            <tr class:orphaned={!!b.orphaned_at} class:other-chain={!!b.chain && !!currentChain && b.chain !== currentChain}>
               <td class="mono">
                 {b.height}
                 {#if b.orphaned_at}
                   <span class="orphan-tag" title="Reorged out of the canonical chain at {b.orphaned_at}">orphaned</span>
                 {/if}
+              </td>
+              <td>
                 {#if b.chain && currentChain && b.chain !== currentChain}
-                  <span class="chain-tag" title="Mined on {b.chain} — current node is {currentChain}">{b.chain}</span>
+                  <span class="chain-tag" title="Mined on {displayChain(b.chain)} - current node is on {displayChain(currentChain)}">{displayChain(b.chain)}</span>
+                {:else if b.chain}
+                  <span class="chain-current">{displayChain(b.chain)}</span>
+                {:else}
+                  <span class="chain-unknown">—</span>
                 {/if}
               </td>
               <td class="hash-cell">
@@ -128,15 +142,24 @@
   }
   .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-size: 0.8em;
     font-weight: 600;
     text-decoration: none;
-    vertical-align: middle;
+  }
+  .chain-current {
+    color: var(--fg-dim);
+    font-size: 0.8em;
+  }
+  .chain-unknown {
+    color: var(--fg-dim);
+    font-size: 0.8em;
+  }
+  tr.other-chain td {
+    opacity: 0.6;
   }
   .unit {
     color: var(--fg-dim);