Fix Zerodha callback state propagation

backend/app/services/zerodha_service.py

@@ -33,7 +33,9 @@ def build_login_url(api_key: str, redirect_url: str | None = None, state: str |
     if redirect_url:
         params["redirect_url"] = redirect_url
     if state:
-        params["state"] = state
+        # Kite Connect echoes redirect_params back to the redirect URL.
+        # Using a raw "state" query parameter here is not reliable.
+        params["redirect_params"] = urllib.parse.urlencode({"state": state})
     query = urllib.parse.urlencode(params)
     return f"{KITE_LOGIN_URL}?{query}"