Add session persistence and browser functionality

- Implement `StorageManager` for saving, loading, and managing sessions.
- Introduce platform-specific session directories for persistence.
- Add session browser UI for listing, loading, and deleting saved sessions.
- Enable AI-generated descriptions for session summaries.
- Update configurations to support storage settings and description generation.
- Extend README and tests to document and validate new functionality.

crates/owlen-core/src/lib.rs

@@ -11,6 +11,7 @@ pub mod model;
 pub mod provider;
 pub mod router;
 pub mod session;
+pub mod storage;
 pub mod types;
 pub mod ui;
 pub mod wrap_cursor;
@@ -54,6 +55,9 @@ pub enum Error {
     #[error("Serialization error: {0}")]
     Serialization(#[from] serde_json::Error),
 
+    #[error("Storage error: {0}")]
+    Storage(String),
+
     #[error("Unknown error: {0}")]
     Unknown(String),
 }