resolve_model_override_key refused the case fallback for every filesystem path,
but only a POSIX path is case-sensitive. A Windows drive path, a UNC share and a
WSL drive path each name one file whatever the casing, and the browser folds
exactly those three before storing. A Windows user's migrated entry was
therefore keyed lowercase while an API auto-switch resolved the same file with
its on-disk casing, so the lookup missed and the saved launch flags silently
stopped applying until the settings were saved again.
Fold those three shapes here too, normalizing the separator as the browser does
so C:/Models/Foo.gguf and c:\models\foo.gguf agree. POSIX stays case-sensitive,
/mnt/data stays an ordinary mount rather than a WSL drive, and an ambiguous fold
still matches nothing so a load takes defaults instead of guessing.
The existing Windows test asserted the opposite. It carried no rationale, unlike
its POSIX sibling, and get_model_override's docstring already scopes the rule to
POSIX, so it read as an over-generalisation of the POSIX case.
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