fix: improve Grafana login routing by checking Referer header

The /login handler now checks both the redirectTo query parameter
and the Referer header to properly detect Grafana login requests.
This fixes the 405 Method Not Allowed error when logging into
Grafana through the dashboard at /monitoring.

Previous behavior:
- Only checked redirectTo query parameter with "goto" string
- POST requests from Grafana login form don't include this parameter
- Requests were incorrectly routed to ChatUI which returns 405

New behavior:
- Checks redirectTo parameter (for GET requests)
- Checks Referer header for "/embedded/grafana" or "/monitoring"
- Properly routes both GET and POST Grafana login requests

Tested with:
- Direct Grafana access: https://grafana-vllm-semantic-router-system.apps.cluster-hw4mm.hw4mm.sandbox590.opentlc.com (working)
- Dashboard Grafana access: https://dashboard-vllm-semantic-router-system.apps.cluster-hw4mm.hw4mm.sandbox590.opentlc.com/monitoring (now working)

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

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

Author: szedan-rh

dashboard/backend/router/router.go

@@ -201,10 +201,18 @@ func Setup(cfg *config.Config) *http.ServeMux {
 			if middleware.HandleCORSPreflight(w, r) {
 				return
 			}
-			// Check if this is a Grafana login redirect by looking at the redirectTo parameter
+			// 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)
 			redirectTo := r.URL.Query().Get("redirectTo")
-			if redirectTo != "" && strings.Contains(redirectTo, "goto") && grafanaStaticProxy != nil {
-				log.Printf("Proxying Grafana login redirect: %s %s (redirectTo=%s)", r.Method, r.URL.Path, redirectTo)
+			referer := r.Header.Get("Referer")
+
+			isGrafanaRequest := (redirectTo != "" && strings.Contains(redirectTo, "goto")) ||
+				strings.Contains(referer, "/embedded/grafana") ||
+				strings.Contains(referer, "/monitoring")
+
+			if isGrafanaRequest && grafanaStaticProxy != nil {
+				log.Printf("Proxying Grafana login: %s %s (redirectTo=%s, referer=%s)", r.Method, r.URL.Path, redirectTo, referer)
 				grafanaStaticProxy.ServeHTTP(w, r)
 				return
 			}