feat: add private key viewing for addresses

Add a 'Show private key' button on the address detail view that opens
a dedicated password-prompt screen with a clear warning about key
sensitivity. After correct password entry, the derived private key is
displayed in a read-only well with a copy button.

- Add getPrivateKeyForAddress() to wallet.js
- Add showPrivateKey view with password verification
- Add clipboard policy section to README explaining why we never
  auto-clear the clipboard
- Register new view in helpers.js VIEWS array and wire up in index.js

Closes #19

src/shared/wallet.js

@@ -41,6 +41,18 @@ function getSignerForAddress(walletData, addrIndex, decryptedSecret) {
     return new Wallet(decryptedSecret);
 }
 
+function getPrivateKeyForAddress(walletData, addrIndex, decryptedSecret) {
+    if (walletData.type === "hd") {
+        const node = HDNodeWallet.fromPhrase(
+            decryptedSecret,
+            "",
+            BIP44_ETH_PATH,
+        );
+        return node.deriveChild(addrIndex).privateKey;
+    }
+    return decryptedSecret;
+}
+
 function isValidMnemonic(mnemonic) {
     return Mnemonic.isValidMnemonic(mnemonic);
 }
@@ -51,5 +63,6 @@ module.exports = {
     hdWalletFromMnemonic,
     addressFromPrivateKey,
     getSignerForAddress,
+    getPrivateKeyForAddress,
     isValidMnemonic,
 };