Fix mnemonic input to not echo to screen

Changed mnemonic input to use secure non-echoing input like passphrases:
- Use secret.ReadPassphrase() instead of readLineFromStdin()
- Add newline after hidden input for better UX
- Remove unused stdin reading functions from cli.go

This prevents sensitive mnemonic phrases from being displayed on screen
during input, matching the security behavior of passphrase input.

internal/cli/init.go

@@ -60,14 +60,17 @@ func (cli *Instance) Init(cmd *cobra.Command) error {
 		mnemonicStr = envMnemonic
 	} else {
 		secret.Debug("Prompting user for mnemonic phrase")
-		// Read mnemonic from stdin using shared line reader
-		var err error
-		mnemonicStr, err = readLineFromStdin("Enter your BIP39 mnemonic phrase: ")
+		// Read mnemonic securely without echo
+		mnemonicBuffer, err := secret.ReadPassphrase("Enter your BIP39 mnemonic phrase: ")
 		if err != nil {
 			secret.Debug("Failed to read mnemonic from stdin", "error", err)
 
 			return fmt.Errorf("failed to read mnemonic: %w", err)
 		}
+		defer mnemonicBuffer.Destroy()
+
+		mnemonicStr = mnemonicBuffer.String()
+		fmt.Fprintln(os.Stderr) // Add newline after hidden input
 	}
 
 	if mnemonicStr == "" {