Harvest integration: per-user OAuth + native timer widget

Harvest time tracking now supports per-user OAuth and uses the Harvest Platform widget for a more native timer experience.

To access this, Enable Harvest in Apps. Connect your Harvest account when prompted (each user connects their own account). From here, start/stop timers from Hive using the integrated Harvest timer experience.

This helps create cleaner individual authentication (no shared credentials) and faster, more familiar Harvest timer flow from within Hive.