feat: allow a single activity proposition and add optional location field

Senders no longer need to fill in two activities to publish; one is
now enough (still capped at 5). Each activity can also specify a
place, which flows into the generated Google Calendar link and .ics
file when the recipient picks a slot.

prisma/schema.prisma

@@ -35,6 +35,7 @@ model ActivityOption {
   proposal    Proposal   @relation(fields: [proposalId], references: [id], onDelete: Cascade)
   title       String
   description String?
+  location    String?
   emoji       String     @default("🎉")
   order       Int        @default(0)
   slots       TimeSlot[]