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/AdminUsersPage.jsx

@@ -16,7 +16,7 @@ export function AdminUsersPage() {
     if (me?.role === "admin") load().catch((e) => setError(String(e.message || e)));
   }, [me]);
 
-  if (me?.role !== "admin") return <div className="card">Nur fuer Admin.</div>;
+  if (me?.role !== "admin") return <div className="card">Admins only.</div>;
 
   const create = async (e) => {
     e.preventDefault();
@@ -47,7 +47,7 @@ export function AdminUsersPage() {
         <input
           type="password"
           value={form.password}
-          placeholder="passwort"
+          placeholder="password"
           onChange={(e) => setForm({ ...form, password: e.target.value })}
         />
         <select value={form.role} onChange={(e) => setForm({ ...form, role: e.target.value })}>
@@ -55,7 +55,7 @@ export function AdminUsersPage() {
           <option value="operator">operator</option>
           <option value="admin">admin</option>
         </select>
-        <button>User anlegen</button>
+        <button>Create user</button>
       </form>
       <div className="card">
         <table>