How can a girl subtly indicate to a righteous young man who seems interested in her that he should propose, especially since he is her neighbor, given her desire to marry to avoid the temptations of the mixed-gender university environment?

There is no harm in a woman offering herself to a man of good character and religion, so that he may propose to her, provided there is no fear of temptation (fitna) and no suspicion. Indeed, Al-Bukhari dedicated a chapter to this. It is permissible to inform the man of this desire through her female or male relatives, or by speaking to him directly, while adhering to Sharia-compliant controls such as avoiding seclusion (khalwa) and lowering the gaze, although mediation is better and further removed from temptation. If there is a fear of temptation from mixed environments, then it is obligatory to remain at home.