fix: 修复@提及uid始终为0的BUG + 空@提示优化

- SearchUsersByLevel SQL id列别名改为uid，修复GORM字段名映射导致uid=0
- @空输入时悬浮窗显示'输入以查找用户'而非空白
- 清理历史脏数据(uid=0的mention记录)

internal/repository/comment_mention_repo.go

@@ -80,7 +80,7 @@ func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int
 	threshold := model.LevelThresholds[minLevel]
 
 	err := r.db.Table("users").
-		Select("id, username, avatar, exp").
+		Select("id AS uid, username, avatar, exp").
 		Where("username LIKE ? AND exp >= ? AND deleted_at IS NULL", "%"+keyword+"%", threshold).
 		Order("exp DESC").
 		Limit(limit).

templates/MetaLab-2026/static/js/post-comments.js

@@ -296,9 +296,9 @@
 
             clearTimeout(searchTimer);
 
-            // 仅输入 @ 时，显示空白下拉
+            // 仅输入 @ 时，显示提示
             if (query.length === 0) {
-                showMention(input, [], '');
+                showEmptyHint(input);
                 return;
             }
 
@@ -366,6 +366,23 @@
         dropdown.style.left = Math.min(pos.left, window.innerWidth - 230) + 'px';
     }
 
+    function showEmptyHint(input) {
+        var dropdown = mentionDropdown;
+        dropdown.innerHTML = '';
+        var hint = document.createElement('div');
+        hint.className = 'mention-no-result';
+        hint.textContent = '输入以查找用户';
+        dropdown.appendChild(hint);
+
+        var cursorIdx = input.selectionStart;
+        var pos = getCursorPixelPos(input, cursorIdx);
+        dropdown.style.display = 'block';
+        dropdown.style.width = '220px';
+        var dropHeight = dropdown.offsetHeight || 36;
+        dropdown.style.top = (pos.top - dropHeight - 4 + window.scrollY) + 'px';
+        dropdown.style.left = Math.min(pos.left, window.innerWidth - 230) + 'px';
+    }
+
     function hideMention() {
         mentionDropdown.style.display = 'none';
         mentionDropdown.innerHTML = '';