feat: implement plugin architecture and application settings with Svelte UI

- Added plugin base and loader for backend extensibility
- Implemented application settings management with config persistence
- Created Svelte-based frontend with Dashboard and Settings pages
- Added API routes for plugins, tasks, and settings
- Updated documentation and specifications
- Improved project structure and developer tools

docs/settings.md

@@ -0,0 +1,46 @@
+# Web Application Settings Mechanism
+
+This document describes the settings management system for the Superset Tools application.
+
+## Overview
+
+The settings mechanism allows users to configure multiple Superset environments and global application settings (like backup storage) via the web UI.
+
+## Backend Architecture
+
+### Data Models
+
+Configuration is structured using Pydantic models in `backend/src/core/config_models.py`:
+
+- `Environment`: Represents a Superset instance (URL, credentials).
+- `GlobalSettings`: Global application parameters (e.g., `backup_path`).
+- `AppConfig`: The root configuration object.
+
+### Configuration Manager
+
+The `ConfigManager` (`backend/src/core/config_manager.py`) handles:
+- Persistence to `config.json`.
+- CRUD operations for environments.
+- Validation and logging.
+
+### API Endpoints
+
+The settings API is available at `/settings`:
+
+- `GET /settings`: Retrieve all settings (passwords are masked).
+- `PATCH /settings/global`: Update global settings.
+- `GET /settings/environments`: List environments.
+- `POST /settings/environments`: Add environment.
+- `PUT /settings/environments/{id}`: Update environment.
+- `DELETE /settings/environments/{id}`: Remove environment.
+- `POST /settings/environments/{id}/test`: Test connection.
+
+## Frontend Implementation
+
+The settings page is located at `frontend/src/pages/Settings.svelte`. It provides forms for managing global settings and Superset environments.
+
+## Integration
+
+Existing plugins and utilities use the `ConfigManager` to fetch configuration:
+- `superset_tool/utils/init_clients.py`: Dynamically initializes Superset clients from the configured environments.
+- `BackupPlugin`: Uses the configured `backup_path` as the default storage location.