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/popup/views/helpers.js

@@ -16,6 +16,7 @@ const VIEWS = [
     "import-key",
     "main",
     "address",
+    "show-private-key",
     "address-token",
     "send",
     "confirm-tx",