refactor: merge workspace (core/ + ui/) into a single flat binary crate

integrate CLI into main.rs, no lib.rs by design (YAGNI)

src/error.rs

@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/error.rs
+//
+// Central error and result types for Noctua.
+
+use std::fmt;
+
+pub type Result<T> = std::result::Result<T, Error>;
+
+#[derive(Debug)]
+pub enum Error {
+    /// A requested file or resource was not found.
+    NotFound(String),
+    /// The file format or operation is not supported.
+    Unsupported(String),
+    /// A session could not be loaded or saved.
+    Session(String),
+    /// An underlying I/O error.
+    Io(std::io::Error),
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Error::NotFound(msg) => write!(f, "Not found: {msg}"),
+            Error::Unsupported(msg) => write!(f, "Unsupported: {msg}"),
+            Error::Session(msg) => write!(f, "Session error: {msg}"),
+            Error::Io(err) => write!(f, "I/O error: {err}"),
+        }
+    }
+}
+
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Error::Io(err) => Some(err),
+            _ => None,
+        }
+    }
+}
+
+impl From<std::io::Error> for Error {
+    fn from(err: std::io::Error) -> Self {
+        Error::Io(err)
+    }
+}