Prevent overwriting changes made on Firefly

[adyanth]

This might be a niche requirement, but if a manual edit is made on Firefly, it should not be overwritten, but if the Splitwise transaction was updated, it should be synced. It can be done using the Splitwise's "updated at" field.

[adyanth]

While at it, we can also delete transactions from Firefly if they were deleted in Splitwise