fix(login): make beginSubmission a single state update

    }

    private fun beginSubmission(): Boolean {

        // Atomic compare-and-set: the first caller to flip isSubmitting false -> true wins and

        // returns true. A read-modify-write on _uiState.value would let two concurrent callers

        // both read isSubmitting=false and both proceed, kicking off duplicate submissions.

        while (true) {

            val currentState = _uiState.value

        if (currentState.isSubmitting) {

            return false

        }

        _uiState.value = currentState.copy(

            if (currentState.isSubmitting) return false

            val nextState = currentState.copy(

                isSubmitting = true,

                errorMessage = null,

                navigationTarget = null,

            )

        return true

            if (_uiState.compareAndSet(currentState, nextState)) return true

        }

    }

    private fun finishSubmission() {