feat: update upload functionality and permissions for document handling

- Removed the `upsert` option from the file upload in `uploadDoc` function to prevent unintended overwrites.
- Enhanced German translations in `i18n.js` for better clarity and consistency in the admin interface.
- Added new CSS styles for link interactions to improve user experience in `styles.css`.
- Updated Supabase SQL migration to grant additional permissions for anonymous users to insert and update storage objects, ensuring proper functionality during the booking flow.

Co-authored-by: Copilot <copilot@github.com>

frontend/nginx-admin.conf

@@ -0,0 +1,26 @@
+server {
+    listen 80;
+    server_name _;
+
+    root /usr/share/nginx/html;
+    index admin.html;
+
+    # Never cache config.js / html so runtime config updates take effect.
+    location = /config.js { add_header Cache-Control "no-store"; try_files $uri =404; }
+    location ~* \.html$    { add_header Cache-Control "no-store"; try_files $uri =404; }
+
+    location / {
+        try_files $uri $uri/ /admin.html;
+    }
+
+    # Static assets: images/fonts can be cached, JS/CSS must revalidate.
+    location ~* \.(?:jpg|jpeg|png|webp|svg|ico|woff2?)$ {
+        expires 7d;
+        add_header Cache-Control "public";
+        try_files $uri =404;
+    }
+    location ~* \.(?:css|js)$ {
+        add_header Cache-Control "no-cache";
+        try_files $uri =404;
+    }
+}