feat(problemset): replace semantic-ui dropdown with native select for hardness filter

- Replace custom semantic-ui dropdown component with standard HTML select element
- Simplify JavaScript by using native event listener instead of semantic-ui dropdown API
- Remove unnecessary dropdown initialization and selection setting code
- Maintain same functionality while reducing complexity and dependency on semantic-ui

web/template/syzoj/problemset.php

@@ -108,39 +108,31 @@
             $selected_hardness = 0;
         }
         ?>
-        <div class="ui selection dropdown" id="hardness_dropdown" style="width: 120px; height: 28px; margin-left: 10px; vertical-align: middle;">
-          <input type="hidden" id="hardness_value" value="<?php echo $selected_hardness; ?>">
-          <i class="dropdown icon"></i>
-          <div class="default text">难度</div>
-          <div class="menu">
-            <div class="item" data-value="0">全部难度</div>
-            <?php for ($h = 1; $h <= 8; $h++) {
-              echo '<div class="item" data-value="' . $h . '">难度 ' . $h . '</div>';
-            } ?>
-          </div>
-        </div>
+        <select id="hardness_select" class="ui dropdown" style="width: 120px; height: 28px; margin-left: 10px; vertical-align: middle;">
+          <option value="0" <?php if ($selected_hardness == 0)
+            echo 'selected'; ?>>全部难度</option>
+          <?php for ($h = 1; $h <= 8; $h++) {
+            echo '<option value="' . $h . '"' . ($selected_hardness == $h ? ' selected' : '') . '>难度 ' . $h . '</option>';
+          } ?>
+        </select>
       </div>
     </div>
   </div>
 
   <script>
     $(function () {
-      $('#hardness_dropdown').dropdown({
-        onChange: function (value) {
-          var params = new URLSearchParams(window.location.search);
-          if (value == '0' || value === '') {
-            params.delete('hardness');
-          } else {
-            params.set('hardness', value);
-          }
-          params.delete('page');
-          window.location.href = '?' + params.toString();
+      document.getElementById('hardness_select').addEventListener('change', function () {
+        var value = this.value;
+        var params = new URLSearchParams(window.location.search);
+        if (value == '0' || value === '') {
+          params.delete('hardness');
+        } else {
+          params.set('hardness', value);
         }
+        params.delete('page');
+        var query = params.toString();
+        window.location.href = query ? ('?' + query) : window.location.pathname;
       });
-      var hv = $('#hardness_value').val();
-      if (hv && hv !== '0') {
-        $('#hardness_dropdown').dropdown('set selected', hv);
-      }
     });
   </script>