Fix path parsing when trailing backslash precedes closing quote on Windows

[joffrey-b]

Summary

On Windows, passing a quoted path with a trailing backslash (e.g. "C:\Music\My Album\") causes rsgain to report a misleading "does not exist" error, with a stray " visible at the end of the path in the error message. This affects both easy mode and custom mode.

Closes #187

Root cause

Windows' CommandLineToArgvW treats \" as an escaped literal " rather than the closing delimiter of a quoted argument. As a result, a path like "C:\Music\My Album\" is parsed by the C runtime as C:\Music\My Album" — the trailing backslash is consumed as an escape character, and the quote becomes part of the string.

This is a common friction point because PowerShell tab-completion for paths with spaces automatically appends a trailing \, making this the default form users encounter.

Fix

Before constructing a std::filesystem::path from a command-line path argument on Windows, strip a trailing " character if present.

Changes

• src/easymode.cpp — strip trailing " from the directory argument before passing to scan_easy()
• src/scan.cpp — strip trailing " from each file path argument before assigning to the filesystem path

Testing

Tested on Windows 11 with PowerShell. The following previously failing invocations now work correctly:

rsgain easy "C:\Music\My Album\"
rsgain easy 'C:\Music\My Album\'