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/LoginPage.jsx

@@ -18,7 +18,7 @@ export function LoginPage() {
       await login(email, password);
       navigate("/");
     } catch {
-      setError("Login fehlgeschlagen");
+      setError("Login failed");
     } finally {
       setLoading(false);
     }
@@ -28,12 +28,12 @@ export function LoginPage() {
     <div className="login-wrap">
       <form className="card login-card" onSubmit={submit}>
         <div className="login-eyebrow">NexaPG Monitor</div>
-        <h2>Willkommen zurück</h2>
-        <p className="login-subtitle">Melde dich an, um Monitoring und Query Insights zu öffnen.</p>
+        <h2>Welcome back</h2>
+        <p className="login-subtitle">Sign in to access monitoring and query insights.</p>
         <div className="input-shell">
           <input
             type="email"
-            placeholder="E-Mail"
+            placeholder="Email"
             value={email}
             onChange={(e) => setEmail(e.target.value)}
             autoComplete="username"
@@ -49,7 +49,7 @@ export function LoginPage() {
           />
         </div>
         {error && <p className="error">{error}</p>}
-        <button className="login-cta" disabled={loading}>{loading ? "Bitte warten..." : "Einloggen"}</button>
+        <button className="login-cta" disabled={loading}>{loading ? "Please wait..." : "Sign in"}</button>
       </form>
     </div>
   );