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/Cargo.toml

@@ -23,6 +23,8 @@ futures = "0.3.28"
 async-trait = "0.1.73"
 toml = "0.8.0"
 shellexpand = "3.1.0"
+dirs = "5.0"
 
 [dev-dependencies]
 tokio-test = { workspace = true }
+tempfile = { workspace = true }