feat(domain): improve domain form UX

- Make name field optional (defaults to domain if empty)
- Simplify domain placeholder and error messages
- Add test connection button for credential validation before save

src/preload/index.ts

@@ -13,6 +13,7 @@ const api: ElectronAPI = {
   updateDomain: (params) => ipcRenderer.invoke("updateDomain", params),
   deleteDomain: (id) => ipcRenderer.invoke("deleteDomain", id),
   testConnection: (id) => ipcRenderer.invoke("testConnection", id),
+  testDomainConnection: (params) => ipcRenderer.invoke("testDomainConnection", params),
 
   // ==================== Browse ====================
   getSpaces: (params) => ipcRenderer.invoke("getSpaces", params),