chore: inital commit

Split project into a Cargo workspace with two crates:

- core/ (noctua_core): UI-independent document library
- ui/ (noctua_ui): COSMIC application scaffold (empty)

justfile for workspace (run, run-cli, check, test, build)

.gitignore

@@ -0,0 +1,14 @@
+.cargo/
+*.pdb
+**/*.rs.bk
+debug/
+target/
+vendor/
+vendor.tar
+debian/*
+!debian/changelog
+!debian/control
+!debian/copyright
+!debian/install
+!debian/rules
+!debian/source