Add optional charset restriction to InputField

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

openvpncertupdate.py

@@ -689,15 +689,16 @@ class InputField:
 
     def __init__(
         self, win, y: int, x: int, width: int,
-        initial: str = "", mask: bool = False,
+        initial: str = "", mask: bool = False, allowed: Optional[str] = None,
     ) -> None:
-        self._win   = win
-        self._y     = y
-        self._x     = x
-        self._width = width
-        self._mask  = mask
-        self._buf   = list(initial)
-        self._cur   = len(self._buf)
+        self._win     = win
+        self._y       = y
+        self._x       = x
+        self._width   = width
+        self._mask    = mask
+        self._allowed = allowed
+        self._buf     = list(initial)
+        self._cur     = len(self._buf)
 
     @property
     def value(self) -> str:
@@ -737,7 +738,10 @@ class InputField:
             if self._cur < len(self._buf):
                 del self._buf[self._cur]
         elif 32 <= key <= 126:
-            self._buf.insert(self._cur, chr(key))
+            ch = chr(key)
+            if self._allowed is not None and ch not in self._allowed:
+                return
+            self._buf.insert(self._cur, ch)
             self._cur += 1