Surface underlying network error instead of generic 'fetch failed'

undici's fetch() collapses DNS/TLS/connection-refused/timeout failures into a vague TypeError. Unwrap err.cause so Test Connection shows the real reason (e.g. ECONNREFUSED, certificate error) instead of just 'fetch failed'.

backend/src/integrations/proxmox.ts

@@ -16,6 +16,17 @@ function authHeader(apiKey: string): Record<string, string> {
 // Proxmox ships with a self-signed cert by default, which Node's fetch rejects out of the box.
 const insecureDispatcher = new Agent({ connect: { rejectUnauthorized: false } })
 
+// undici's fetch() wraps the real failure (DNS, TLS, connection refused/timed out) in a generic
+// "fetch failed" TypeError — unwrap err.cause so the actual reason reaches the UI.
+function describeFetchError(err: unknown): string {
+  if (err instanceof Error) {
+    const cause = (err as { cause?: unknown }).cause
+    if (cause instanceof Error) return `${err.message}: ${cause.message}`
+    return err.message
+  }
+  return 'Connection failed'
+}
+
 export const proxmox: IntegrationAdapter = {
   async testConnection(config, secrets) {
     const baseUrl = config.baseUrl?.replace(/\/$/, '')
@@ -30,7 +41,7 @@ export const proxmox: IntegrationAdapter = {
       if (!res.ok) return { ok: false, message: `HTTP ${res.status}` }
       return { ok: true, message: 'Connected' }
     } catch (err) {
-      return { ok: false, message: err instanceof Error ? err.message : 'Connection failed' }
+      return { ok: false, message: describeFetchError(err) }
     }
   },