Implement internationalization support across components

- Integrated `next-intl` for toast messages and locale handling in various components, including `Providers`, `CanvasUserMenu`, and `CreditOverview`.
- Replaced hardcoded strings with translation keys to enhance localization capabilities.
- Updated `RootLayout` to dynamically set the language attribute based on the user's locale.
- Ensured consistent user feedback through localized toast messages in actions such as sign-out, canvas operations, and billing notifications.

components/locale-switcher.tsx

@@ -0,0 +1,24 @@
+'use client';
+import { useLocale } from 'next-intl';
+import { useRouter } from 'next/navigation';
+
+export function LocaleSwitcher() {
+  const locale = useLocale();
+  const router = useRouter();
+
+  function switchLocale() {
+    const next = locale === 'de' ? 'en' : 'de';
+    document.cookie = `NEXT_LOCALE=${next}; path=/; max-age=31536000; SameSite=Lax`;
+    router.refresh();
+  }
+
+  return (
+    <button
+      onClick={switchLocale}
+      className="text-xs text-muted-foreground hover:text-foreground transition-colors"
+      title={locale === 'de' ? 'Switch to English' : 'Auf Deutsch wechseln'}
+    >
+      {locale === 'de' ? 'EN' : 'DE'}
+    </button>
+  );
+}