[BUILD-937] fix: Reviewer UI fixes

[RisingOrange]

Related issues

https://ankihub.atlassian.net/jira/software/c/projects/BUILD/boards/1?selectedIssue=BUILD-937

Proposed changes

• fix: Use correct font for empty state page
  • Here is the design for the no-content page: https://www.figma.com/design/rWzQ7PPqAXf1piz5yTntaT/%F0%9F%91%91--Ankihub?node-id=8006-27532&t=F2Hz9n6TdGOZ6XWe-1
• Don't set up sidebar if mh_integration flag is not active
  • If you just have the chatbot feature flag and not the mh_integration feature flag, the reviewer buttons are still set up, this should not be the case
  • 
• B&B, FA, chatbot buttons
  • Don't show chatbot for decks without embeddings
  • Hide buttonsButton if there are no other visible buttons
    • Fixes this issue:
    
  • Adjust button border radius to 4px
  • Change button color on hover
• Header
  • Make tabs have same width and truncate long tab titles
  • Change on-hover style of tabs
  • Only enable the open-in-browser button on non-AnkiHub pages

Comments

• I tried making cursor: pointer work for some elements, but it seems like this is an issue related to qwebengine