Rename `clientName` to `platformName`

[fwcd]

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"