feat: add admin-only git pull interface and remove auto-pull

- Add new `gitpull.php` page for manual git pull execution by administrators
- Remove automatic git pull on homepage visit to prevent unintended updates
- Include git pull link in admin navigation menu for easy access
- Secure interface with admin-only access and sudo credentials configuration

web/template/syzoj/header.php

@@ -210,6 +210,9 @@
                                 <?php } ?>
                             <?php if(isset($_SESSION[$OJ_NAME.'_'.'administrator'])||isset($_SESSION[$OJ_NAME.'_'.'contest_creator'])||isset($_SESSION[$OJ_NAME.'_'.'problem_editor'])){ ?>
                             <a class="item" href="admin/"><i class="settings icon"></i><?php echo $MSG_ADMIN;?></a>
+                            <?php } ?>
+                            <?php if(isset($_SESSION[$OJ_NAME.'_'.'administrator'])){ ?>
+                            <a class="item" href="gitpull.php"><i class="cloud download icon"></i>更新系统</a>
                             <?php }
 if(isset($_SESSION[$OJ_NAME.'_'.'balloon'])){
   echo "<a class=item href='balloon.php'><i class='golf ball icon'></i>$MSG_BALLOON</a>";