fix: glitchtip errors (locale, email) + E2E submission cleanup

- Add fallback 'de' locale to toLocaleDateString() to prevent RangeError
- Skip sending emails for submissions from 'testing@mintel.me'
- Update check-forms.ts to automatically delete test submissions via Payload API
- (Manual) Configured MAIL_FROM and MAIL_RECIPIENTS on alpha.mintel.me

components/home/RecentPosts.tsx

@@ -74,7 +74,7 @@ export default async function RecentPosts({ locale, data }: RecentPostsProps) {
                       suppressHydrationWarning
                       className="px-3 py-1 text-white/80 text-[10px] md:text-xs font-bold uppercase tracking-widest border border-white/20 rounded-full bg-white/10 backdrop-blur-md"
                     >
-                      {new Date(post.frontmatter.date).toLocaleDateString(locale, {
+                      {new Date(post.frontmatter.date).toLocaleDateString(locale || 'de', {
                         year: 'numeric',
                         month: 'short',
                         day: 'numeric',