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/ui.rs

@@ -22,6 +22,7 @@ pub enum InputMode {
     Help,
     Visual,
     Command,
+    SessionBrowser,
 }
 
 impl fmt::Display for InputMode {
@@ -34,6 +35,7 @@ impl fmt::Display for InputMode {
             InputMode::Help => "Help",
             InputMode::Visual => "Visual",
             InputMode::Command => "Command",
+            InputMode::SessionBrowser => "Sessions",
         };
         f.write_str(label)
     }