Preserve case when loading firmware from WSL UNC shares

[Prasana-sys]

Selecting firmware from a WSL share on Windows can fail with “Specified firmware file is not readable”: os.path.normcase() changes a directory such as FirmwareProject to firmwareproject, which does not exist on the case-sensitive Linux filesystem. The file-server widget repeats the same conversion after selection.

Add a shared normalize_local_path() helper and use it for firmware selection, FileServerWidget.add_path, and PathItem.path. Preserve case for both \\wsl.localhost\ and \\wsl$\, including Qt-style forward slashes, while retaining existing normalization for ordinary Windows paths, generic UNC paths, and POSIX paths. The existing short-key firmware protocol stays unchanged.

Add 23 pytest cases covering path normalization and the firmware-selection-to-file-read flow, and run them in the existing Python package workflow.

Validation:

• All 23 tests pass on Windows with Python 3.12.14, DroneCAN 1.0.27, and PyQt6 6.11.0.
• Restoring the original widget code reproduces both WSL integration failures; the local-file case still passes.
• Real Windows-to-WSL testing reproduces FileNotFoundError before the fix in both namespaces. After the fix, selection, short-key resolution, GetInfo, and 895 chunks totaling 228,892 bytes succeed with matching SHA-256 hashes.
• The patch applies cleanly to the base commit and all 23 tests pass again on that clean copy.

The CAN node was simulated; no physical device was flashed. Legacy non-key directory lookup in the DroneCAN dependency is outside this firmware-update fix.