[Fix] Chat controls fade into light IDE themes (#1298)

* fix(webview): adapt chat controls to IDE themes

* test(webview): cover theme-aware control states

* test(webview): add visual regression screenshots for theme-aware chat controls

---------

Co-authored-by: Roomote <roomote@roomote.dev>
Co-authored-by: Elliott de Launay <edelaunay@wealthsimple.com>