[BUILD-465] feat: Send theme setting for chatbot to iframe

We want the theme of the chatbot to be adapted to Anki's theme. This PR implements this together with this web app PR: https://github.com/ankipalace/ankihub/pull/2179

Related issues

Proposed changes

Send theme setting to the iframe using postMessage

How to reproduce

Checkout the web app PR: https://github.com/ankipalace/ankihub/pull/2179
Comment out this code to make the chatbot show up for any AnkiHub deck: https://github.com/ankipalace/ankihub_addon/blob/1a8b0eb4b3c2b030b0c538c44fea8fc6014e151f/ankihub/gui/reviewer.py#L104-L107
Enable the chatbot feature flag on your local ankihub web app
Open an ankihub note in the reviewer
Follow the steps in the video

Screenshots and videos

https://github.com/ankipalace/ankihub_addon/assets/31575114/a7d9b63b-0d57-47a9-9131-274ecf7aa942

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.73%. Comparing base (5f5b58f) to head (f41fcf2).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #984 +/- ## ======================================= Coverage 87.73% 87.73% ======================================= Files 66 66 Lines 6758 6761 +3 ======================================= + Hits 5929 5932 +3 Misses 829 829 ```

AnkiHubSoftware / ankihub_addon