Migrate from Vite to Next.js 15 with SSR

- Replace Vite + React Router with Next.js 15 App Router
- Implement i18n with next-intl (URL-based: /de, /en, /sr)
- Add SSR/SSG for all pages (48 static pages generated)
- Setup Supabase SSR client for auth
- Migrate all pages: Home, About, Portfolio, Blog, Contact, Login, Dashboard, Imprint, Privacy, Terms
- Add Docker support with standalone output
- Replace i18next with next-intl JSON translations
- Use next/image for optimized images

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/i18n/config.ts

@@ -0,0 +1,16 @@
+export const locales = ['de', 'en', 'sr'] as const;
+export const defaultLocale = 'de' as const;
+
+export type Locale = (typeof locales)[number];
+
+export const localeNames: Record<Locale, string> = {
+  de: 'Deutsch',
+  en: 'English',
+  sr: 'Srpski',
+};
+
+export const localeFlags: Record<Locale, string> = {
+  de: '🇩🇪',
+  en: '🇬🇧',
+  sr: '🇷🇸',
+};