feat: add session import/export functionality

- Add JSON export with full session metadata for backup/restore
- Add Markdown export with formatted conversation history
- Add import capability for previously exported sessions
- Add tauri-plugin-fs for file system operations
- Add export dropdown menu and import button to session history panel

src-tauri/Cargo.toml

@@ -27,6 +27,7 @@ tauri-plugin-notification = "2"
 tauri-plugin-os = "2"
 tauri-plugin-http = "2"
 tauri-plugin-clipboard-manager = "2"
+tauri-plugin-fs = "2"
 tempfile = "3"
 semver = "1"
 chrono = { version = "0.4.43", features = ["serde"] }