feat: add password visible button

lib/RegisterPage.dart

@@ -23,6 +23,7 @@ class _RegisterPageState extends State<RegisterPage> {
   final TextEditingController _passwordController = TextEditingController();
   final TextEditingController _codeController = TextEditingController();
 
+  bool _passwordNotVisible = true;
   bool _isButtonEnabled = true;
   int _seconds = 60;
   Timer? _timer;
@@ -228,9 +229,21 @@ class _RegisterPageState extends State<RegisterPage> {
                     decoration: InputDecoration(
                       border: OutlineInputBorder(),
                       prefixIcon: Icon(Icons.lock_outlined),
+                      suffixIcon: IconButton(
+                        icon: Icon(_passwordNotVisible
+                            ? Icons.visibility
+                            : Icons.visibility_off),
+                        onPressed: () {
+                          setState(
+                                () {
+                              _passwordNotVisible = !_passwordNotVisible;
+                            },
+                          );
+                        },
+                      ),
                       labelText: '密碼',
                     ),
-                    obscureText: true,
+                    obscureText: _passwordNotVisible,
                   ),
                   SizedBox(height: 20),
                   Row(