Fix Block model to match server: nest header with camelCase keys

The block endpoint returns block id / prev / timestamp nested under a
"header" object with camelCase keys (blockId, dcId, prevId, prevProof,
timestamp) and a proof of just {proof}. The previous flat snake_case
Block fields never matched the response and always deserialized empty.
Add a BlockHeader interface, nest it in Block, and make proof.scheme
optional. Verified live against a dev chain.

src/types.ts

@@ -122,17 +122,22 @@ export interface SmartContract {
 
 // Block Types
 export interface BlockProof {
-  scheme: string;
   proof: string;
+  scheme?: string; // absent on trust-scheme chains; present on PoW
   nonce?: number;
 }
 
+export interface BlockHeader {
+  blockId: string;
+  dcId: string;
+  prevId: string;
+  prevProof: string;
+  timestamp: string;
+}
+
 export interface Block {
   version: string;
-  block_id: string;
-  timestamp: string;
-  prev_id: string;
-  prev_proof: string;
+  header: BlockHeader;
   transactions: string[];
   proof: BlockProof;
 }