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/app.js

@@ -494,7 +494,7 @@ async function uploadDoc(leadId, file, kind) {
         const path = `${leadId}/${kind}.${ext}`;
         const { error: upErr } = await supabase.storage
             .from("customer-documents")
-            .upload(path, file, { contentType: file.type, upsert: true });
+            .upload(path, file, { contentType: file.type });
         if (upErr) { console.error("Upload failed:", upErr); return; }
         await supabase.from("lead_attachments").insert({
             lead_id: leadId,