feat(message-input): add search box to slash-command popups

Both the inline autocomplete (triggered by `/` in the textarea) and the
dropdown popup (triggered by the slash-command button) now show a search
field at the top. Matching uses substring on name and description, and
ranks name matches above description/id-only matches.

src/i18n/messages/zh-CN.json

@@ -1573,7 +1573,9 @@
         "forkAndSend": "分叉发送",
         "slashCommands": "斜杠命令",
         "expertSkills": "专家技能",
-        "expertsEmptyForAgent": "该智能体没有启用任何专家。请在「设置 > 专家」中启用。"
+        "expertsEmptyForAgent": "该智能体没有启用任何专家。请在「设置 > 专家」中启用。",
+        "slashSearchPlaceholder": "搜索命令...",
+        "slashSearchEmpty": "没有匹配的命令"
       },
       "messageQueue": {
         "addToQueue": "加入队列",