fix(ci): Always use main branch for assistant app

[mejo-]

The assistant app doesn follow the branch strategy of server.

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.31%. Comparing base (ac33b1d) to head (b29a62b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7723   +/-   ##
=======================================
  Coverage   59.31%   59.31%           
=======================================
  Files         503      503           
  Lines       39258    39258           
  Branches     1130     1130           
=======================================
  Hits        23285    23285           
  Misses      15865    15865           
  Partials      108      108           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.