feat: add local file storage and stdio transport support

[matteoredaelli]

Summary

This PR adds two features to make the ECharts MCP server easier to use in local/development environments:

1. Local file storage (default)

• Adds a STORAGE_MODE env variable (local | cloud)
• Local mode (default): saves generated chart images to ./images/ directory — no cloud credentials needed
• Cloud mode: uses BaiduBCE BOS (existing behavior), now lazily loaded only when needed
• Removes the mandatory .env file check, enabling zero-config local usage

2. Stdio transport

• Adds --stdio flag to run the server with stdio transport instead of SSE/HTTP
• Adds start:stdio npm script for convenience
• Enables direct integration with LLM clients (e.g. Claude Desktop, VS Code extensions) without requiring a running HTTP server

Other improvements

• Improved date formatting in generated file names (zero-padded)
• Better error messages when cloud credentials are missing

Usage

# Local mode with stdio (zero config)
npm run start:stdio

# Local mode with SSE (zero config)
npm run dev

# Cloud mode (requires BOS credentials in .env)
STORAGE_MODE=cloud npm run dev

No breaking changes — existing cloud setups continue to work by setting STORAGE_MODE=cloud.