refactor: make cache_manager thread safe

[TheBobBobs]

Summary

Fixes errors from multiple render threads adding/removing cache folders.
Fixes register_existing_folders only calculating the size for the last folder in the cache.

Tasks Completed

• Platforms Tested:
  • Windows x86
  • Windows ARM
  • macOS x86
  • macOS ARM
  • Linux x86
  • Linux ARM
• Tested For:
  • Basic functionality
  • PyInstaller executable

[CyanVoxel]

Thank you for opening this PR, the wonky threading is an internal issue of the current cache system that I've been wanting to have fixed since it was introduced.

However I am currently unable to review this further as it crashes when opening most libraries for me (macOS 15.5):
(There may be multiple issues occurring here)

2025-08-19 13:54:06 [info     ] [FFmpeg] Using FFprobe location: ffprobe (Found)
2025-08-19 13:54:06 [info     ] [FFmpeg] Using FFmpeg location: ffmpeg (Found)
2025-08-19 13:54:06 [info     ] [ResourceManager] Resources Registered: count=33
2025-08-19 13:54:06 [info     ] [Settings] Global Settings File Path not specified, using default
2025-08-19 13:54:06 [info     ] [Settings] Reading Global Settings File path=PosixPath('/Users/cyanvoxel/.config/TagStudio/settings.toml')
2025-08-19 13:54:06 [info     ] [Cache] Cache File not specified, using default one filename=/Users/cyanvoxel/.config/TagStudio/TagStudio.ini
2025-08-19 13:54:08 [error    ] [Errno 20] Not a directory: '/Users/cyanvoxel/Local Files/Projects/TagStudio Project/Test Libraries/Example 2/.TagStudio/thumbs/.DS_Store'
Traceback (most recent call last):
  File "/Users/cyanvoxel/Local Files/Dev/TagStudio/src/tagstudio/main.py", line 64, in main
    driver.start()
  File "/Users/cyanvoxel/Local Files/Dev/TagStudio/src/tagstudio/qt/ts_qt.py", line 556, in start
    self.open_library(path_result.library_path)
  File "/Users/cyanvoxel/Local Files/Dev/TagStudio/src/tagstudio/qt/ts_qt.py", line 1672, in open_library
    defaultValue=self.lib.cache_manager.max_size,
                 ^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Library' object has no attribute 'cache_manager'
2025-08-19 13:54:08 [info     ] 
TagStudio Frontend (Qt) Crashed! Press Enter to Continue...

I am also able to "jam up" the cache deletion by tossing in a random file into the thumbs folder, causing a traceback. From what I can tell you're choosing to delete the entire thumbs folder now instead of each of the folders inside it, which is contributing to this problem.

[TheBobBobs]

This should now work with random files in the cache folder. As well as cache folders being deleted outside of tagstudio.
I ended up just removing the bytes to megabytes conversion.

[CyanVoxel]

Everything seems to be functional now, thank you for your work on this! I've just resolved the merge conflicts with #1057 and will get this merged.