chore: Migrate from Makefile to justfile

[FreddyFunk]

same story like my other pull requests for migrations to just.

[jacobgkau]

This builds fine locally, but failed to build on the build server.

Build server output:

   Compiling xdg-desktop-portal-cosmic v0.1.0 (/<<PKGBUILDDIR>>)
warning: fields `handle`, `app_id`, and `parent_window` are never read
   --> src/file_chooser.rs:246:9
    |
245 | pub(crate) struct Args {
    |                   ---- fields in this struct
246 |     pub handle: zvariant::ObjectPath<'static>,
    |         ^^^^^^
247 |     pub app_id: String,
    |         ^^^^^^
248 |     pub parent_window: String,
    |         ^^^^^^^^^^^^^
    |
    = note: `Args` has derived impls for the traits `Debug` and `Clone`, but these are intentionally ignored during dead code analysis
    = note: `#[warn(dead_code)]` on by default

warning: methods `feedback_formats` and `modifiers_for_format` are never used
  --> src/wayland/mod.rs:65:8
   |
58 | impl DmabufHelper {
   | ----------------- methods in this implementation
...
65 |     fn feedback_formats(&self) -> impl Iterator<Item = &DmabufFormat> {
   |        ^^^^^^^^^^^^^^^^
...
73 |     pub fn modifiers_for_format(&self, format: u32) -> impl Iterator<Item = u64> + '_ {
   |            ^^^^^^^^^^^^^^^^^^^^

E: ABORT: Received TERM signal (requesting cleanup and shutdown)
E: Caught signal ‘Terminated’: terminating immediately
error: Recipe `build-debug` was terminated on line 38 by signal 15
make[1]: *** [debian/rules:20: override_dh_auto_build] Error 143
make: *** [debian/rules:8: build] Terminated
--------------------------------------------------------------------------------
Build finished at 2026-02-11T21:47:14Z

Finished
--------

Not sure what's causing this, as the warnings don't look like they should necessarily cause a build failure.