Rename `clientName` to `platformName`

This would improve clarity, as a follow-up to #140. The reason this was left out of the PR is that we'd need to:

- Perform the rename in code
- Add a migration mechanism to automatically migrate the `local` folder to `platformName`
- Perhaps write a small script that only renames JSON keys (`clientName` -> `platformName`) and invoke it at startup?
- Maybe we could collect migration scripts in a `Migrations` folder, named by their version and then apply them in order
- We'd probably want some form of versioning, e.g. a `local/.version.json` that records the "schema version"