Add "Open local folder" and "Create new vault" options to the vault
picker dropdown. Uses tauri-plugin-dialog for native folder picker
in Tauri mode, falls back to prompt() in browser mode.
- Rust: add tauri-plugin-dialog, create_vault_dir command
- Frontend: pickFolder utility, StatusBar UI, App.tsx handlers
- Mock: create_vault_dir mock handler for browser testing
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>