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)

ui/src/main.rs

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/main.rs
+//
+// Entry point for the Noctua COSMIC application.
+
+fn main() {
+    // TODO: Initialize the COSMIC application.
+}