4chan.org support

Seems possible. Have to find way to convert Base64 md5 hash in browser to generic md5, add backend support for 4chan links and finally add 4chan support in extension (the most tedious).

Start by tracing the backend link handling and the browser extension entry point, then investigate how the Base64 MD5 hash used by 4chan can be converted to a generic MD5 value. Done means 4chan links are recognized and supported by both the backend and the extension, with the hash conversion working in the browser.

Written by the indexing model from the issue text.