build: migrate to uv, standardize packging

[mokurin000]

Sorry for this PR without an issue, I will create an issue later when I am available

For short, the nonstandard source tree annoyed me for fifteen minutes figuring out failed to find module "backend" by directory or filename

The output of poetry install:

Warning: The current project could not be installed: No file/folder found for package backend
If you do not want to install the current project use --no-root.
If you want to use Poetry only for dependency management but not for packaging, you can disable package mode by setting package-mode = false in your pyproject.toml file.

In a future version of Poetry this warning will become an error!

Changes/Reason

1. Standardize packaging

This will fix the above problem, in the best way
Also, previously we use absolute module name to import a local module, which will make the program only able to be executed at backend/

2. Migrate to uv

Poetry does not support standard project metadata format at all.
It will just throw:

[tool.poetry] section not found in C:/Users/<retracted>/Documents/Repos/screenshot-to-code/backend/pyproject.toml 

PDM is also applicable to applications.

3. Build backend changed to setuptools (default fallback)

This is mainly a side-effect of 1.. But we need no plugins support, no C/C++/Rust Extensions, so setuptools is just enough