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.

pkg/database/get-serials-by-range.go

@@ -31,9 +31,9 @@ func (d *D) GetSerialsByRange(idx Range) (
 			for it.Seek(endBoundary); it.Valid(); it.Next() {
 				item := it.Item()
 				var key []byte
-				key = item.Key()[:len(key)-5]
+				key = item.Key()
 				if bytes.Compare(
-					key, idx.Start,
+					key[:len(key)-5], idx.Start,
 				) < 0 {
 					// didn't find it within the timestamp range
 					return