feat: add lesson export and import functionality

- Implemented `LessonExport` and `LessonImport` actions in the action module.
- Added corresponding command parsing for lesson export and import.
- Created functions to handle lesson export and import in the memory module.
- Updated state management to reflect changes after lesson operations.
- Introduced deferred operations for handling asynchronous tasks in the event loop.
- Enhanced the tool execution context to include graduated checks for file operations.
- Added OAuth support with PKCE for secure authorization flows.
- Implemented a loopback server for handling OAuth redirects.
- Refactored various modules to improve code organization and maintainability.

src/service/oauth/pkce.rs

@@ -0,0 +1,41 @@
+use base64::{Engine as _, engine::general_purpose::URL_SAFE_NO_PAD};
+use sha2::{Sha256, Digest};
+
+const VERIFIER_LENGTH: usize = 64;
+
+pub struct CodeVerifier(String);
+
+impl CodeVerifier {
+    pub fn new() -> Self {
+        let bytes: Vec<u8> = (0..VERIFIER_LENGTH).map(|_| rand_byte()).collect();
+        CodeVerifier(URL_SAFE_NO_PAD.encode(&bytes))
+    }
+
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+
+    pub fn challenge(&self) -> CodeChallenge {
+        let mut hasher = Sha256::new();
+        hasher.update(self.0.as_bytes());
+        let digest = hasher.finalize();
+        CodeChallenge(URL_SAFE_NO_PAD.encode(digest))
+    }
+}
+
+fn rand_byte() -> u8 {
+    use std::time::{SystemTime, UNIX_EPOCH};
+    let nanos = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .unwrap_or_default()
+        .subsec_nanos();
+    (nanos & 0xFF) as u8
+}
+
+pub struct CodeChallenge(String);
+
+impl CodeChallenge {
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+}