Basic auth parsing crashes on malformed headers

[hourianto]

https://github.com/planety/prologue/blob/devel/src/prologue/auth/auth.nim#L34
https://github.com/planety/prologue/blob/devel/src/prologue/auth/auth.nim#L52

Index into split results without checking length. Inputs like Authorization: Basic or a base64 payload without : raise IndexDefect, which becomes a 500 via the global handler rather than a 401, and will be undefined behavior with -d:release

Found by GPT 5.4