fix: build hashes and recovery-phrase entropy come from ethers (closes #758)

build.js and script/lib/package.js hash with sha256 from ethers, with the
0x prefix stripped so the build receipt and SHA256SUMS keep bare lowercase
hex. generateMnemonic() takes its 16 bytes of entropy from randomBytes in
ethers. The README Crypto policy no longer lists these as exceptions.

Model: opus-5-5

src/shared/wallet.js

@@ -1,7 +1,7 @@
 // Wallet operations: mnemonic generation, HD derivation, signing.
 // All crypto delegated to ethers.js.
 
-const { Mnemonic, HDNodeWallet, Wallet } = require("ethers");
+const { Mnemonic, HDNodeWallet, Wallet, randomBytes } = require("ethers");
 const { DEBUG, DEBUG_MNEMONIC, BIP44_ETH_PATH } = require("./constants");
 
 function generateMnemonic() {
@@ -10,10 +10,7 @@ function generateMnemonic() {
     // settings toggle drives, which would let a user of a release build turn
     // the hardcoded, publicly known test phrase back on for real wallets.
     if (DEBUG) return DEBUG_MNEMONIC;
-    const m = Mnemonic.fromEntropy(
-        globalThis.crypto.getRandomValues(new Uint8Array(16)),
-    );
-    return m.phrase;
+    return Mnemonic.fromEntropy(randomBytes(16)).phrase;
 }
 
 // Every extended key (xprv or xpub) entering the app goes through this.