Dev server bug: `No theme candidate in asset files, but Gitea requires there should be at least one usable theme`

[silverwind]

1. Run make watch
2. Open http://localhost:3000/admin in the browser when backend build has finished but before webpack has finished
3. The themes in the instance are broken and the admin page displays this error. Also the theme selector in the footer is broken.

The root cause is that themes are currently only read once on app startup. We need to change the backend to read themes dynamically.

For ideal performance, this live reading should only be done in development. In production mode continue to read only once.

Gitea Version

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