Added "Open for coffee" notification

So, quick update here:

basically the idea is, founders set open for coffee = on if they want to meet others in person.

Then, when a founder joins or is travelling in their city, both will receive an email and/or X post and DM.

I added this feature as I saw some founders using FoundersAround to meet each other. Especially digital nomads, solo founders. We feel isolated and want to connect.