Standardize English language usage and improve environment configuration

Replaced German text with English across the frontend UI for consistency and accessibility. Enhanced clarity in `.env.example` and `README.md`, adding detailed comments for environment variables and prerequisites. Improved documentation for setup, security, and troubleshooting.

frontend/src/pages/DashboardPage.jsx

@@ -26,7 +26,7 @@ export function DashboardPage() {
     };
   }, [tokens, refresh]);
 
-  if (loading) return <div className="card">Lade Dashboard...</div>;
+  if (loading) return <div className="card">Loading dashboard...</div>;
   if (error) return <div className="card error">{error}</div>;
 
   return (
@@ -54,7 +54,7 @@ export function DashboardPage() {
               <th>Name</th>
               <th>Host</th>
               <th>DB</th>
-              <th>Aktion</th>
+              <th>Action</th>
             </tr>
           </thead>
           <tbody>