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.lock

@@ -1613,6 +1613,7 @@ dependencies = [
  "tauri-build",
  "tauri-plugin-clipboard-manager",
  "tauri-plugin-dialog",
+ "tauri-plugin-fs",
  "tauri-plugin-http",
  "tauri-plugin-notification",
  "tauri-plugin-opener",