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/styles.css

@@ -900,6 +900,9 @@ table.admin-table tbody tr:hover {
     transform: translateX(4px);
 }
 
+.link-lead { text-decoration: none; cursor: pointer; }
+.link-lead:hover code { color: var(--accent-strong); text-decoration: underline; }
+
 .admin-form { display: grid; gap: 1rem; }
 .admin-form label { display: grid; gap: 0.3rem; font-size: 0.85rem; color: var(--muted); transition: color 0.2s; }
 .admin-form label:focus-within { color: var(--accent-strong); }