Refactor backupCreate for testability

[ivanauth]

This PR makes the backup creation code easier to test by extracting dependencies.

Changes

• Extract backupCreateImpl that accepts a client and progress tracker as dependencies
• Add ProgressTracker interface to separate progress file logic from backup logic
• Now possible to test edge cases like PermissionDenied without spinning up a full server

Also fixes a time.Tick goroutine leak along the way.

Resolves #569

[codecov-commenter]

Codecov Report

❌ Patch coverage is 64.13043% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.18%. Comparing base (7ed5ab0) to head (8437759).
⚠️ Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/cmd/backup.go	64.13%	18 Missing and 15 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #604      +/-   ##
==========================================
+ Coverage   39.28%   42.18%   +2.90%     
==========================================
  Files          37       37              
  Lines        5448     4805     -643     
==========================================
- Hits         2140     2027     -113     
+ Misses       3063     2520     -543     
- Partials      245      258      +13     

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