hotfix/shixiang/new_version_nodejs

[tangshixiang]

Summary

Fix npm install failure caused by the native canvas package requiring system-level C++ libraries (Cairo, Pango, etc.) that may not be present on all machines. Make canvas optional with a graceful runtime fallback.

Changes

Move canvas from dependencies to optionalDependencies in package.json so npm install no longer fails when native build tools are missing
Refactor pdf-image-extractor.ts to use lazy dynamic import("canvas") instead of a static import, with automatic fallback to text-only PDF extraction when canvas is unavailable
Export isCanvasAvailable() helper for callers that need to check canvas availability
Add "Optional: PDF Image Extraction (canvas)" section to README.md with system dependency install commands for macOS, Ubuntu/Debian, Fedora/RHEL, and Windows

Related Issues

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Checklist

• My code follows the project's coding style
• I have added tests that cover my changes (if applicable)
• All new and existing tests pass (npm test)
• Lint passes (npm run lint)
• I have updated the documentation (if applicable)
• I have checked that there are no sensitive data (API keys, tokens, etc.) in my changes