feat(resume): add progress bar when checking existing data (#1031)

Fixes #1026

When resuming a large file transfer (>10MB), display a progress bar
showing 'Checking [filename]' while scanning the existing file to
determine which chunks are missing.

This provides visual feedback during the check phase, preventing
users from thinking the application has frozen on large files.

Co-authored-by: Murat Aslan <murataslan1@users.noreply.github.com>