feat: enhance math rendering and copy functionality in chat

[chenyme]

Summary

Fix Markdown math rendering in chat messages and improve formula readability/interaction. The update makes Streamdown math rendering more robust for \(...\) / \[...\], table cells containing LaTeX absolute-value pipes such as |x|, and inline math split across lines. It also initializes the math plugin eagerly to avoid refresh-time render misses, adds formula source copy support, applies consistent KaTeX typography, and tightens spacing around rendered dividers/headings.

Change type

• Bug fix
• Feature
• Documentation
• Refactor
• Configuration / deployment
• Security hardening
• Other

Affected areas

• Frontend / UI
• Backend / API
• Authentication / authorization
• Conversations / streaming
• Files / RAG / extraction
• Model routing / providers
• MCP / tools
• Billing / payments
• Admin console
• Deployment / Docker / configuration
• Documentation

Verification

• pnpm --dir frontend lint
• pnpm --dir frontend exec tsc --noEmit
• git diff --check

Screenshots, API examples, or logs

Tested with Markdown table examples containing LaTeX formulas such as \ln(1+x) and |x| < 1. The formulas now render through KaTeX instead of being split by Markdown table parsing.

Configuration, migration, and compatibility notes

Configuration changes: None.

API changes: None.

Database migrations: None.

Deployment changes: None.

Backward compatibility: Existing conversation content remains unchanged; rendering is improved at the frontend Markdown layer.

Documentation

• Documentation is not needed for this change.
• Documentation was updated.
• Documentation still needs to be updated.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• User data access remains scoped by authenticated user context unless an admin-only path explicitly requires broader access.
• Security-sensitive behavior was reviewed, including authentication, authorization, provider routing, file processing, billing, and admin APIs where relevant.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior, deployment steps, API contracts, or configuration changes are documented.
• Generated artifacts are included only when this project explicitly requires them.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.