fix: add checkmark prefix to created/synced output lines and fix progress bar counter

[greghuels]

Checkmarks were missing for both flags created and synced to indicate that the submission was successful. I had this at one point, but I think it regressed after the React refactor.

Summary

• Prefix successful Created and Synced output lines with a green checkmark so they are visually distinct from the overwrite warnings that precede them
• Fix the progress bar checkmark counter to track both created and synced flags (was only counting created, showing 0 when all flags were syncs)
• Rename ProgressBarStats.created to saved to reflect that it represents all successful submissions

Test plan

• Run a migration with a mix of new and existing flags and verify checkmark appears on each successful line
• Confirm the progress bar checkmark counter increments for both created and synced flags