fix(mcp): improve error message internationalization

- Add translateMcpBackendError utility to map backend errors to i18n keys
- Update error handling in McpPanel, McpFormModal, and McpWizardModal
- Internationalize stdio/http type selectors in Wizard
- Implement three-tier fallback strategy: translation → raw error → default message
- No backend changes required, fully frontend-based i18n implementation

src/i18n/locales/en.json

@@ -293,6 +293,8 @@
       "title": "MCP Configuration Wizard",
       "hint": "Quickly configure MCP server and auto-generate JSON configuration",
       "type": "Type",
+      "typeStdio": "stdio",
+      "typeHttp": "http",
       "command": "Command",
       "commandPlaceholder": "npx or uvx",
       "args": "Arguments",

src/i18n/locales/zh.json

@@ -293,6 +293,8 @@
       "title": "MCP 配置向导",
       "hint": "快速配置 MCP 服务器，自动生成 JSON 配置",
       "type": "类型",
+      "typeStdio": "stdio",
+      "typeHttp": "http",
       "command": "命令",
       "commandPlaceholder": "npx 或 uvx",
       "args": "参数",