Add Discord forum registry sync

[Ku-Tadao]

Summary

Adds an automated Discord forum sync for the community plugin store.

Changes

• Adds a scheduled/manual GitHub Action that runs every 30 minutes.
• Reads the plugin and theme Discord forum channels with a bot token stored as \DISCORD_BOT_TOKEN.
• Extracts GitHub/release links from the first 5 messages of each forum thread.
• Enriches listings with GitHub repo metadata, latest release info, preview images, tags, author info, and download assets.
• Writes generated
  egistry/store.json\ for the app and
  egistry/store.yml\ for human review.
• Adds a GitHub Pages deploy workflow that publishes the existing site plus
  egistry/, so Pengu can fetch \https://penguloader.github.io/plugin-store/registry/store.json\.

Validation

• Ran \pnpm install --frozen-lockfile.
• Ran \pnpm sync:discord-store\ locally and generated 46 listings.
• Ran \pnpm build\ successfully.

Notes

The repository needs a \DISCORD_BOT_TOKEN\ Actions secret before the scheduled sync can run in GitHub Actions. The PenguLoader app PR can use the raw fork branch as a temporary fallback while this PR is under review.