Add internationalization support with English/German language toggle across all components

Integrated i18n system throughout the application with `useLanguage` hook providing translation function and language state. Added bilingual support to all user-facing strings in App.jsx (password validation, error messages, game creation, sheet sections), AdminPanel (user management, form validation, status messages), AdminSettingsModal (SMTP configuration), ChipModal, DesignModal, GamePickerCard (lobby interface,

frontend/src/components/GameStartCelebration.jsx

@@ -2,8 +2,10 @@ import React, { useEffect } from "react";
 import { createPortal } from "react-dom";
 import confetti from "canvas-confetti";
 import { stylesTokens } from "../styles/theme";
+import { useLanguage } from "../i18n";
 
 export default function GameStartCelebration({ open, onClose }) {
+  const { t, language } = useLanguage();
   useEffect(() => {
     if (!open) return;
 
@@ -29,14 +31,14 @@ export default function GameStartCelebration({ open, onClose }) {
   if (!open) return null;
 
   return createPortal(
-    <div className="hp-start-overlay" onClick={onClose} role="dialog" aria-label="Spiel gestartet">
+    <div className="hp-start-overlay" onClick={onClose} role="dialog" aria-label={t("gameStarted")}>
       <div className="hp-start-card" onClick={(event) => event.stopPropagation()}>
         <div className="hp-start-rune">✦</div>
-        <div className="hp-start-kicker">Die Ermittlungen beginnen</div>
-        <div className="hp-start-title">Spiel gestartet</div>
-        <div className="hp-start-subtitle">Möge der beste Detektiv gewinnen.</div>
+        <div className="hp-start-kicker">{t("investigationsBegin")}</div>
+        <div className="hp-start-title">{t("gameStarted")}</div>
+        <div className="hp-start-subtitle">{t("bestDetective")}</div>
         <button onClick={onClose} style={{ marginTop: 18, color: stylesTokens.textGold }}>
-          Los geht&apos;s
+          {language === "en" ? "Let’s go" : "Los geht's"}
         </button>
       </div>
     </div>,