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/login.html

@@ -17,12 +17,14 @@
 
     <form id="loginForm" autocomplete="on">
       <div class="field">
-        <label for="host">Proxmox Host</label>
+        <label for="host">Proxmox Host(s)</label>
         <input type="text" id="host" placeholder="pve.example.com" autocomplete="url" spellcheck="false">
+        <div class="field-hint">Mehrere eigenständige Hosts mit denselben Zugangsdaten? Mit Komma trennen, z. B. pve1.example.com, pve2.example.com</div>
       </div>
       <div class="field">
         <label for="username">Benutzer</label>
-        <input type="text" id="username" placeholder="user@pam" autocomplete="username" spellcheck="false">
+        <input type="text" id="username" placeholder="root@pam" autocomplete="username" spellcheck="false">
+        <div class="field-hint">Format: Benutzer@Realm — z. B. root@pam</div>
       </div>
       <div class="field">
         <label for="password">Passwort</label>