Participant id=1 was named "Me" in the DB, causing Sonu and other users
to see "Me" referring to Siddharth when viewing splits and shared expenses.
- Rename participant id=1 from "Me" to "Siddharth" in the DB
- /api/participants now substitutes "Me" for whichever participant matches
the current user, so the label is always relative to the viewer
- split-modal: default split uses currentUser.id instead of name === "Me"
- transactions/page: filter and display logic uses participant ID not name
- shared/page: split chips show "Me" when participant_id === current user
Also includes add-transaction-modal tags support (pre-existing staged change).