Open folders passed as launch arguments

[rthw]

Open folders passed as launch arguments

Fixes #1176.

RapidRAW previously treated every positional launch argument and macOS open -a RapidRAW <path> request as an image file. Passing a folder therefore opened it in the external editor flow and failed with “Is a directory”.

This change distinguishes directories from files in the launch request path. A folder is added to the library roots when necessary and selected in the library. If it is already inside an existing root, it is selected without creating a duplicate root.

Folder paths are canonicalized (symlinks and trailing separators included), so the same folder is never added twice, and on a cold start the library roots are seeded from the settings before the folder is used — opening a folder no longer replaces the user's existing roots.

How to test

1. Build RapidRAW on macOS.
2. Run open -a RapidRAW /path/to/photo-folder while RapidRAW is closed. The app should open the folder in the library grid.
3. While the app is running, run open -a RapidRAW /path/to/another-folder. The window should come forward and switch to that folder.
4. Run the app binary directly with a folder argument:

   RapidRAW.app/Contents/MacOS/RapidRAW /path/to/photo-folder

5. Repeat step 2 with a folder nested under an existing root. It must not add a second root.
6. Launch with a folder while other library roots are configured: the existing roots must survive.

Not changed

• File arguments still use the existing external image-open flow.
• --edit / --output external edit sessions are unchanged.
• Headless export and normal launches without arguments are unchanged.