Support multiple standalone Proxmox hosts with shared credentials

Comma-separated host list logs into every host, merges VM lists tagged
by origin host, and routes SPICE connections to the right one. Also
adds a username format hint (user@realm) on the login form.

src/renderer/styles.css

@@ -76,6 +76,13 @@ body.login-page {
   margin-bottom: 0.4rem;
 }
 
+.field-hint {
+  font-size: 0.72rem;
+  color: var(--muted);
+  margin-top: 0.35rem;
+  line-height: 1.4;
+}
+
 input[type="text"],
 input[type="password"] {
   width: 100%;