What does it mean for a girl to send a message saying Happy Tanabata?

1 implies that she likes you. According to common sense, if a girl has a lover, she will not wish others happiness on Tanabata, but only the person she likes, Tanabata. If you like her too, you can boldly tell her. If you don't like her, you can not reply, or you can definitely reply to her "I wish you find your one soon", and she will understand.

2. She sends you a Happy Tanabata, perhaps because she wants you to say Happy Tanabata to her, or because she wants you to spend Tanabata with this traditional Valentine's Day in China.

Maybe it's just a general holiday greeting. If you just send these happy holidays, don't think too much. She might send it in groups.

There are two meanings, one is that I like you, and the other is that you two have a very good relationship. Her best wishes to your friends.