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