After upgrading to Mastodon v4.5+ which switched from Webpack to Vite,
the CDN_HOST environment variable was no longer being respected for
Vite-generated assets. This caused all dynamically imported JavaScript
chunks, emoji data files, and other assets to be loaded from the local
domain instead of the configured CDN.

Root cause: Vite's base path was hardcoded to `/${outDirName}/` in
vite.config.mts, which made all asset URLs relative to the local domain.
Vite generates these URLs at build time using import.meta.glob() and
code splitting, so they don't respect Rails' runtime config.asset_host.

Solution: Configure Vite's base option to use CDN_HOST during production
builds. The base path now uses the full CDN URL when CDN_HOST is set
and isProdBuild is true, falling back to the relative path otherwise.

This fixes asset loading for:
- JavaScript chunks (e.g., schedule_idle_task-*.js)
- Emoji data files (e.g., compact-*.json)
- CSS files and other Vite-managed assets

Note: CDN_HOST must be set as an environment variable during the
production build, not just at runtime:
  export CDN_HOST=https://mastodon-static.theatl.social
  yarn build:production

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Consolidate asset copying in Dockerfile.

This passes the CDN_HOST variable to the Docker build so that Vite
can generate asset URLs with the CDN domain during compilation.

Without this, the Dockerfile receives CDN_HOST but the workflow
wasn't passing it, so assets were still using relative paths.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>