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/QueryInsightsPage.jsx

@@ -36,7 +36,7 @@ export function QueryInsightsPage() {
   return (
     <div>
       <h2>Query Insights</h2>
-      <p>Hinweis: Benötigt aktivierte Extension <code>pg_stat_statements</code> auf dem Zielsystem.</p>
+      <p>Note: This section requires the <code>pg_stat_statements</code> extension on the monitored target.</p>
       {error && <div className="card error">{error}</div>}
       <div className="card">
         <label>Target </label>