Various fixes and design updates.

backend/src/api/blocks.ts

@@ -64,7 +64,7 @@ class Blocks {
 
         transactions.sort((a, b) => b.feePerVsize - a.feePerVsize);
         block.medianFee = transactions.length ? this.median(transactions.map((tx) => tx.feePerVsize)) : 0;
-        block.feeRange = transactions.length ? this.getFeesInRange(transactions, 8) : [];
+        block.feeRange = transactions.length ? this.getFeesInRange(transactions, 8) : [0, 0];
 
         this.blocks.push(block);
         if (this.blocks.length > config.KEEP_BLOCK_AMOUNT) {