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)

core/src/document/store.rs

@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/document/store.rs
+//
+// RON-based persistence for session data: load and save session lists.
+//
+// For the session concept and the meaning of each field see `session.rs`.
+//
+// On disk a session file contains a RON list of SessionData records:
+//
+// [
+//     (
+//         name: "Pictures",
+//         kind: DirectoryBrowser,
+//         path: Some("/home/user/Pictures"),
+//         items: [],
+//         current_index: None,
+//     ),
+//     (
+//         name: "Project Docs",
+//         kind: DocumentCollection,
+//         path: None,
+//         items: [
+//             (path: "/home/user/docs/spec.pdf",  page_index: 0),
+//             (path: "/home/user/docs/notes.pdf", page_index: 2),
+//         ],
+//         current_index: Some(0),
+//     ),
+// ]
+
+use crate::document::session::SessionData;
+use crate::error::{Error, Result};
+use std::path::Path;
+
+/// Loads all sessions from a RON file.
+pub fn load_sessions(path: &Path) -> Result<Vec<SessionData>> {
+    let content = std::fs::read_to_string(path).map_err(Error::Io)?;
+
+    ron::from_str(&content)
+        .map_err(|e| Error::Session(format!("Failed to parse session file {:?}: {}", path, e)))
+}
+
+/// Saves all sessions to a RON file.
+#[allow(dead_code)]
+pub fn save_sessions(path: &Path, sessions: &[SessionData]) -> Result<()> {
+    let content = ron::ser::to_string_pretty(sessions, ron::ser::PrettyConfig::default())
+        .map_err(|e| Error::Session(format!("Failed to serialize sessions: {}", e)))?;
+
+    std::fs::write(path, content).map_err(Error::Io)
+}