fix(gui): change app.lock location

[tristanmsct]

Hi

I did not find an issue for this small problem so I created a standalone PR
I'm trying to follow the contributing guidelines
This PR is really simple but this file has been bugging me, I'm sorry if I missed anything in terms of formating or type of commit, etc. Let me know if I need to correct anything

Motivation

The app creates a $XDG_DATA_DIR/my_app.lock when running to know if an instance is already running.
I think the name and place of the file can be improved.

The name of the file does not hint at which program creates it.
The file itself would be better placed in the XDG_RUNTIME_DIR within a nmrs subfolder. Or at least if it stays in $XDG_DATA_DIR, then it can be in an nmrs subfolder.

Description

This PR simply tries to create the lock file in $XDG_RUNTIME_DIR instead of $XDG_DATA_DIR, and if it fails then it falls back to the old directory.
Also the lock file is now nmrs/app.lock instead of just my_app.lock which is clearer for the user.
The changes are only in this file nmrs-gui/src/file_lock.rs.

Testing

• cargo test --all-features : all tests were ok except 15 tests that were ignored.
• cargo test --test integration_test --all-features : 43 tests, all ok

I build the app and tested it, the lock file is created at /run/usr/1000/nmrs/app.lock.
I could not get the creation at this specific place to fail, to confirm that it would indeed be created at ~/.local/share/nmrs/app.lock but I see no reason why it would not work.

AI Disclosure

I am not a rust developer and used a LLM to check my syntaxe and be sure to use the right functions for the job.

[cachebag]

@tristanmsct
Thanks for this! Always appreciate someone cleaning up my code lol.

Just make sure you run fmt and clippy and whatnot and amend your changes to your commit. Appreciate the patch!

also, let's add a test. at minimum one that calls acquire_app_lock() and asserts it succeeds, and a second call that asserts it fails with the "already running" error while the first lock is held. the existing tests in the project should give you a feel for the style.

for the commit message, please just write fix(gui): ...