fix(opencode): address issues found during OpenCode integration review

- Fix MCP server not removed from opencode.json when unchecked in edit modal
- Fix Windows atomic write failure when opencode.json already exists
- Fix i18n keys mismatch in OpenCodeFormFields (use opencode.* namespace)
- Fix unit test missing apps.opencode field assertion

tests/components/McpFormModal.test.tsx

@@ -432,6 +432,7 @@ type = "stdio"
       claude: false,
       codex: false,
       gemini: false,
+      opencode: false,
     });
     expect(onSave).toHaveBeenCalledTimes(1);
     expect(toastErrorMock).not.toHaveBeenCalled();