Prevent system sleep while transcriptions are queued

[wenhaoone]

Summary

• keep Windows and macOS awake while file transcriptions remain queued or running
• add an enabled-by-default General preference to control sleep prevention
• serialize queue activity across threads and make completion/cancellation cleanup stale-safe
• add regression coverage for duplicate task IDs, forced cancellation, cross-thread ordering, and platform adapters

Testing

• 51 passed across sleep inhibitor, queue worker, preferences, and settings tests
• cross-thread MainWindow regression test passed
• Windows native SetThreadExecutionState acquire/release smoke test passed
• all 15 gettext catalogs compiled successfully
• ruff check passed for the new module and tests
• git diff --check passed

The full local suite was also attempted; its first end-to-end transcription case could not complete because this Windows environment does not have the CI-provided ffmpeg executable. The focused and integration tests covering this change pass.

Closes #1422.

[raivisdejus]

Nicely done @wenhaoone!
Thanks for your contribution

[codecov]

Codecov Report

❌ Patch coverage is 92.16867% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.60%. Comparing base (646c61b) to head (8a16724).

Files with missing lines	Patch %	Lines
buzz/sleep_inhibitor.py	90.19%	10 Missing ⚠️
buzz/widgets/main_window.py	77.77%	2 Missing ⚠️
buzz/file_transcriber_queue_worker.py	97.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1593      +/-   ##
==========================================
+ Coverage   83.46%   83.60%   +0.13%     
==========================================
  Files         110      111       +1     
  Lines       12472    12625     +153     
==========================================
+ Hits        10410    10555     +145     
- Misses       2062     2070       +8     

Flag	Coverage Δ
Linux	82.73% <91.56%> (+0.12%)	⬆️
macOS	83.11% <90.36%> (+0.09%)	⬆️

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