feat(mcp): show inline duplicate ID error and block submit in add mode

- Display red hint next to title when ID exists

- Disable Add/Save button and prevent submit on duplicate

- Accept existing IDs via prop for real-time validation

- Remove overwrite confirmation dialog on add

- i18n: add duplicate-ID error strings and remove unused overwrite prompt

- files:

  - src/components/mcp/McpFormModal.tsx

  - src/components/mcp/McpPanel.tsx

  - src/i18n/locales/en.json

  - src/i18n/locales/zh.json

src/i18n/locales/en.json

@@ -316,6 +316,7 @@
     },
     "error": {
       "idRequired": "Please enter identifier",
+      "idExists": "Identifier already exists. Please choose another.",
       "jsonInvalid": "Invalid JSON format",
       "commandRequired": "Please enter command",
       "singleServerObjectRequired": "Please paste a single MCP server object (do not include top-level mcpServers)",

src/i18n/locales/zh.json

@@ -316,6 +316,7 @@
     },
     "error": {
       "idRequired": "请填写标识",
+      "idExists": "该标识已存在，请更换",
       "jsonInvalid": "JSON 格式错误，请检查",
       "commandRequired": "请填写命令",
       "singleServerObjectRequired": "此处只需单个服务器对象，请不要粘贴包含 mcpServers 的整份配置",