fix: add Content-Type checking for Grafana login routing

Enhanced the /login handler to check Content-Type header in addition
to redirectTo parameter and Referer header. Grafana sends login requests
with Content-Type: application/json, while ChatUI uses form data.

This fixes the 405 Method Not Allowed error for Grafana login POST requests.

Note: Grafana still needs GF_SERVER_ROOT_URL configuration to work
properly in iframe, which is a separate deployment configuration issue.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: szedan-rh

dashboard/backend/router/router.go

@@ -203,20 +203,23 @@ func Setup(cfg *config.Config) *http.ServeMux {
 			}
 			// Check if this is a Grafana login request by:
 			// 1. Query parameter redirectTo with "goto" (GET requests)
-			// 2. Referer header containing "/embedded/grafana" or "/monitoring" (POST requests)
+			// 2. Referer header containing "/embedded/grafana" or "/monitoring"
+			// 3. Content-Type: application/json (Grafana uses JSON for login POST)
 			redirectTo := r.URL.Query().Get("redirectTo")
 			referer := r.Header.Get("Referer")
+			contentType := r.Header.Get("Content-Type")
 
 			isGrafanaRequest := (redirectTo != "" && strings.Contains(redirectTo, "goto")) ||
 				strings.Contains(referer, "/embedded/grafana") ||
-				strings.Contains(referer, "/monitoring")
+				strings.Contains(referer, "/monitoring") ||
+				strings.Contains(contentType, "application/json")
 
 			if isGrafanaRequest && grafanaStaticProxy != nil {
-				log.Printf("Proxying Grafana login: %s %s (redirectTo=%s, referer=%s)", r.Method, r.URL.Path, redirectTo, referer)
+				log.Printf("Proxying Grafana login: %s %s (redirectTo=%s, referer=%s, contentType=%s)", r.Method, r.URL.Path, redirectTo, referer, contentType)
 				grafanaStaticProxy.ServeHTTP(w, r)
 				return
 			}
-			log.Printf("Proxying Chat UI login: %s %s", r.Method, r.URL.Path)
+			log.Printf("Proxying Chat UI login: %s %s (contentType=%s)", r.Method, r.URL.Path, contentType)
 			chatUIProxy.ServeHTTP(w, r)
 		})
 		mux.HandleFunc("/logout", func(w http.ResponseWriter, r *http.Request) {