Normalize firmware DB paths to POSIX format for cross-platform compatibility

[Copilot]

When the same firmware folder is accessed from both Windows and Linux/WSL2, Path.relative_to() produces OS-native separators (\ vs /). This causes clean_downloaded_firmwares() to fail matching DB records against disk files, incorrectly deleting valid records.

Changes

• mpflash/download/from_web.py: Use .as_posix() instead of str() when storing the relative path after download
• mpflash/downloaded.py: Use .as_posix() when building the on-disk file set for comparison with DB records

Both db/loader.py and custom/naming.py already used .as_posix() — these two call sites were the only ones missing normalization.

# Before (OS-dependent separators)
board.firmware_file = str(filename.relative_to(firmware_folder))

# After (always forward slashes)
board.firmware_file = filename.relative_to(firmware_folder).as_posix()

Tests

Added two tests validating that POSIX paths are used consistently for DB storage and disk-vs-DB comparison.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.58%. Comparing base (c5bed8f) to head (0977cb2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
+ Coverage   75.55%   75.58%   +0.03%     
==========================================
  Files          54       54              
  Lines        3031     3031              
  Branches      488      488              
==========================================
+ Hits         2290     2291       +1     
  Misses        621      621              
+ Partials      120      119       -1     

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