Improve atlas feedback and rendering UX

[ChatGPU]

Summary

• Add a GitHub Issue based feedback workflow, contribution notes, and contributor attribution metadata.
• Vendor the browser dependencies locally and harden Markdown math rendering so mixed Chinese/English prose and LaTeX render reliably without CDN access.
• Add formula source copy, theme toggle, selected-node focus dimming, and wider compare mode with section alignment plus synced scrolling.
• Fix SVG text encoding and make graph validation use UTF-8 on Windows.

Validation

• python tools\validate_graph.py
• python tools\check_links.py
• node --check docs\app.js docs\compare.js docs\timeline.js
• Edge/Playwright smoke: 75 nodes, 137 KaTeX fragments, 0 rendering failures, compare alignment jump, mobile overflow, SVG MIME/text