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

app/[locale]/blog/[slug]/page.tsx

@@ -113,7 +113,7 @@ export default async function BlogPost({ params }: BlogPostProps) {
                 </Heading>
                 <div className="flex flex-wrap items-center gap-6 text-white/80 text-sm md:text-base font-medium">
                   <time dateTime={post.frontmatter.date} suppressHydrationWarning>
-                    {new Date(post.frontmatter.date).toLocaleDateString(locale, {
+                    {new Date(post.frontmatter.date).toLocaleDateString(locale || 'de', {
                       year: 'numeric',
                       month: 'long',
                       day: 'numeric',
@@ -150,7 +150,7 @@ export default async function BlogPost({ params }: BlogPostProps) {
             </Heading>
             <div className="flex items-center gap-6 text-text-primary/80 font-medium">
               <time dateTime={post.frontmatter.date} suppressHydrationWarning>
-                {new Date(post.frontmatter.date).toLocaleDateString(locale, {
+                {new Date(post.frontmatter.date).toLocaleDateString(locale || 'de', {
                   year: 'numeric',
                   month: 'long',
                   day: 'numeric',

app/[locale]/blog/page.tsx

@@ -198,7 +198,7 @@ export default async function BlogIndex({ params }: BlogIndexProps) {
 
                       <div className="flex items-center gap-3 text-xs md:text-sm font-bold text-white/80 mb-3 tracking-widest uppercase">
                         <time dateTime={post.frontmatter.date} suppressHydrationWarning>
-                          {new Date(post.frontmatter.date).toLocaleDateString(locale, {
+                          {new Date(post.frontmatter.date).toLocaleDateString(locale || 'de', {
                             year: 'numeric',
                             month: 'long',
                             day: 'numeric',