New Features

Sync Log
- Every synchronization is now fully logged: start, copy errors, and completion summary
- Up to 200 entries are stored per sync task (persisted across restarts)
- New "Log" button on each sync card — opens the log view
- Entries are color-coded by severity: green (info), red (error)
- Red ! badge on the Log button when the last sync encountered errors
- Log can be manually cleared

Version in Header
- The popup header now shows version and build number (e.g. v0.2.0 (Build 2))
- The build number increments automatically on every ./build.sh run

Technical

- Logs are stored per sync ID in messenger.storage.local (syncLog_<id>)
- Deleting a sync task also deletes its associated log