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/cli.go

@@ -2,21 +2,11 @@
 package cli
 
 import (
-	"bufio"
-	"fmt"
-	"os"
-	"strings"
-	"syscall"
-
 	"git.eeqj.de/sneak/secret/internal/secret"
 	"github.com/spf13/afero"
 	"github.com/spf13/cobra"
-	"golang.org/x/term"
 )
 
-// Global scanner for consistent stdin reading
-var stdinScanner *bufio.Scanner //nolint:gochecknoglobals // Needed for consistent stdin handling
-
 // Instance encapsulates all CLI functionality and state
 type Instance struct {
 	fs       afero.Fs
@@ -67,33 +57,3 @@ func (cli *Instance) SetStateDir(stateDir string) {
 func (cli *Instance) GetStateDir() string {
 	return cli.stateDir
 }
-
-// getStdinScanner returns a shared scanner for stdin to avoid buffering issues
-func getStdinScanner() *bufio.Scanner {
-	if stdinScanner == nil {
-		stdinScanner = bufio.NewScanner(os.Stdin)
-	}
-
-	return stdinScanner
-}
-
-// readLineFromStdin reads a single line from stdin with a prompt
-// Uses a shared scanner to avoid buffering issues between multiple calls
-func readLineFromStdin(prompt string) (string, error) {
-	// Check if stderr is a terminal - if not, we can't prompt interactively
-	if !term.IsTerminal(syscall.Stderr) {
-		return "", fmt.Errorf("cannot prompt for input: stderr is not a terminal (running in non-interactive mode)")
-	}
-
-	fmt.Fprint(os.Stderr, prompt) // Write prompt to stderr, not stdout
-	scanner := getStdinScanner()
-	if !scanner.Scan() {
-		if err := scanner.Err(); err != nil {
-			return "", fmt.Errorf("failed to read from stdin: %w", err)
-		}
-
-		return "", fmt.Errorf("failed to read from stdin: EOF")
-	}
-
-	return strings.TrimSpace(scanner.Text()), nil
-}