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)

Cargo.toml

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+# Cargo.toml
+#
+# Workspace manifest for the Noctua project.
+
+[workspace]
+members = ["core", "ui"]
+resolver = "2"