feat: upgrade to v1.5.3 with lazy export plugins

[mlightcad]

Summary

• Upgrade to @mlightcad/cad-simple-viewer v1.5.3 and register lazy HTML, PDF, and SVG export plugins from @mlightcad/cad-html-plugin, @mlightcad/cad-pdf-plugin, and @mlightcad/cad-svg-plugin.
• Add export toolbar (HTML / PDF / SVG) after a DXF/DWG file is opened successfully; hide export buttons on the home screen.
• Hide Open and New controls while a file is loading; restore them if open fails.
• Copy viewer-runtime.iife.js from cad-html-plugin for offline HTML export.
• Rewrite README with plugin integration docs and switch the build script to tsc.

Test plan

• pnpm install && pnpm build succeeds
• pnpm dev — home page shows only Open and New
• Open a .dxf or .dwg — Open/New hide during load; export buttons appear after success
• Export HTML, PDF, and SVG (first run may load plugin chunks)
• New drawing still works; export buttons stay hidden until a file is opened
• Failed file open restores Open and New on the home screen