Make ESC clear password field instead of canceling prompt

Change the behavior of the ESC key to clear the password input field
(similar to Ctrl-U) instead of immediately canceling the authentication
prompt. This provides better UX - users who accidentally start typing
their password can press ESC to clear it and try again, rather than
having the prompt close.

Users can still cancel the prompt by waiting for the timeout, and the
previous commit ensures timeout/cancellation won't count as a failed
login attempt.

Related to #114

Amp-Thread-ID: https://ampcode.com/threads/T-60336806-26ca-40b3-bb77-86a913f74a0c
Co-authored-by: Amp <amp@ampcode.com>

Author: Radosław Stachowiak

helpers/auth_x11.c

@@ -1306,8 +1306,14 @@ int Prompt(const char *msg, char **response, int echo) {
           BumpDisplayMarker(priv.pwlen, &priv.displaymarker,
                             &priv.last_keystroke);
           break;
-        case 0:       // Shouldn't happen.
         case '\033':  // Escape.
+          // Clear the input line (like Ctrl-U). User can still press ESC
+          // again or wait for timeout to cancel the prompt entirely.
+          priv.pwlen = 0;
+          BumpDisplayMarker(priv.pwlen, &priv.displaymarker,
+                            &priv.last_keystroke);
+          break;
+        case 0:  // Shouldn't happen.
           done = 1;
           break;
         case '\r':  // Return.