Revert ephemeral event handling changes that broke relaytester

- Remove ephemeral event handling in handle-event.go
- Remove ephemeral event rejection in save-event.go
- Remove formatTimestamp function and title attributes in App.svelte
- Remove TestEphemeralEventRejection test
- Fix slice bounds bug in get-serials-by-range.go
- Restore correct error message format for existing events
- Revert version from v0.13.2 to v0.12.3

This reverts commit 075838150d which introduced
a critical bug causing runtime panics in the relaytester.

app/handle-event.go

@@ -207,12 +207,6 @@ func (l *Listener) HandleEvent(msg []byte) (err error) {
 			}
 		}
 	}
-
-	// Check if the event is ephemeral (kinds 20000-29999) and just deliver to subscribers.
-	if kind.IsEphemeral(env.E.Kind) {
-		go l.publishers.Deliver(env.E)
-		return
-	}
 	// store the event - use a separate context to prevent cancellation issues
 	saveCtx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
 	defer cancel()