feat: add GFM alerts support in markdown preview

[touero]

Summary

This PR adds support for GitHub Flavored Markdown (GFM) Alerts in markdown-preview.nvim rendering.
close #632, close #663, close #715

Supported syntax examples:

Note

This is a note.

Warning

This is a warning.

What Changed

• Added GFM alert parsing in the Markdown rendering pipeline.
• Added alert styles (light/dark theme tokens + block/title styles).
• Added dependency declaration for markdown-it-github-alerts.
• Added documentation updates in README (feature list + references).
• Updated exported frontend assets under app/out/*.

Files changed

• app/pages/index.jsx
  • Registered alert plugin in markdown-it chain.
  • Switched to a local compatibility wrapper import.
• app/pages/markdown-it-github-alerts-compat.js (new)
  • Added compatibility wrapper adapted from markdown-it-github-alerts (MIT attribution included).
• app/_static/markdown.css
  • Added alert color variables for light/dark themes.
  • Added .markdown-alert* styles.
• package.json
  • Added markdown-it-github-alerts dependency (pinned for compatibility).
• yarn.lock
  • Updated lockfile for the new dependency.
• README.md
  • Added GFM Alerts to features and references.
• app/out/*
  • Regenerated static export artifacts.

Implementation Details

The target app uses an older build stack (Next 7 / webpack 4).
Directly importing newer package builds caused parse/build issues in this environment, so this PR uses a compatibility wrapper in app/
pages/markdown-it-github-alerts-compat.js, adapted from markdown-it-github-alerts behavior, and wires it into the existing markdown-it
plugin chain.

This keeps the rendering behavior aligned with GitHub-style alerts while remaining compatible with the current project toolchain.

Verification

• Build succeeded with legacy OpenSSL flag:
  • NODE_OPTIONS=--openssl-legacy-provider yarn build-app
• Alert rendering check passed:
  • generated HTML contains markdown-alert markdown-alert-note
  • generated HTML contains markdown-alert-title
• Manual preview validation:
  • Opened preview and confirmed NOTE/WARNING blocks render as styled alert boxes.

Notes

• This PR intentionally keeps compatibility with the current legacy frontend build toolchain.
• Wrapper source attribution is included in the compat file header (MIT upstream).