feat(FN-1899): implement login timeout, cancellation, and 409 conflict handling

- Add login outcome types (timeout, success, failed) and state tracking via stepData
- Implement login timeout after MAX_POLL_CYCLES (150 polls × 2s = 5 minutes) with warning toast
- Add 409 Conflict detection for concurrent login attempts with warning toast
- Add cancellation capability for in-progress logins with cleanup and state reset
- Update ModelOnboardingModal tests to cover timeout and concurrent login scenarios

packages/dashboard/app/components/__tests__/ModelOnboardingModal.test.tsx

@@ -1321,7 +1321,7 @@ describe("ModelOnboardingModal", () => {
       await waitFor(() => {
         expect(addToast).toHaveBeenCalledWith(
           "Login already in progress. Please wait or cancel the current attempt.",
-          "info"
+          "warning"
         );
       });
     });
@@ -1517,7 +1517,7 @@ describe("ModelOnboardingModal", () => {
       });
 
       // Should show timeout toast
-      expect(addToast).toHaveBeenCalledWith("Login timed out. Please try again.", "info");
+      expect(addToast).toHaveBeenCalledWith("Login timed out. Please try again.", "warning");
 
       // Cancel button should not be shown after timeout
       expect(screen.queryByText("Cancel")).toBeNull();