Fix: Remove X-Frame-Options header from Collabora proxy to allow iframe loading

- The collaboraProxyHandler endpoint is intentionally meant to load in an iframe
- X-Frame-Options: SAMEORIGIN was blocking cross-origin iframe loads
- Removed the header to allow the Flutter web frontend to embed the proxy page

go_cloud/internal/http/wopi_handlers.go

@@ -689,7 +689,7 @@ func collaboraProxyHandler(w http.ResponseWriter, r *http.Request, db *database.
 </html>`, collaboraURL, wopiSrc)
 
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
-	w.Header().Set("X-Frame-Options", "SAMEORIGIN")
+	// Don't set X-Frame-Options - this endpoint is meant to be loaded in an iframe
 	w.WriteHeader(http.StatusOK)
 	w.Write([]byte(htmlContent))