Fix network diff adjustment prediction algo

ui/src/lib/PoolOverview.svelte

@@ -36,12 +36,16 @@
   });
 
   // Blocks remaining + ETA until the retarget. The predicted adjustment
-  // percent comes from the backend (computed from the epoch start time).
+  // percent comes from the backend (measured between block timestamps
+  // from the epoch's first block), as does the epoch's observed mean
+  // block time — projecting the ETA at the pace the network is actually
+  // keeping rather than a flat 600s, which is what mempool.space shows.
   const retarget = $derived.by(() => {
     if (!height) return { remaining: 2016, eta: "", progress: 0 };
     const inEpoch = height % 2016;
     const remaining = 2016 - inEpoch;
-    const etaSec = remaining * 600;
+    const avgBlockSec = data.epoch_avg_block_seconds || 600;
+    const etaSec = remaining * avgBlockSec;
     const progress = (inEpoch / 2016) * 100;
     return { remaining, eta: formatDuration(etaSec), progress };
   });

ui/src/types.ts

@@ -166,6 +166,9 @@ export type Snapshot = {
   cumulative_shares: number;
   next_block_reward_btc: number;
   next_difficulty_percent: number;
+  // Observed mean seconds per block so far this epoch, measured between
+  // block timestamps. Absent before the first block of an epoch.
+  epoch_avg_block_seconds?: number;
   chain: BlockchainInfo | null;
   network_hashrate_hs: number;
   recent_blocks?: BlockRecord[];