WHAT DOES THIS DO?

(Fixes issue: "Add ability to purge historical sessions in batch​ #2603 ")

Adds new section with a "select" button that when clicked, you can select threads to bulk delete them.

Note: Frontend implementation can be changed based on feedback. Maybe instead of a visible button add it on contextual menu in the three-dot menu to start the "selection" process. Not a professional frontend guy tho

Note: Three dot menu for threads are now permanently visible on mobile view too, which couldn't be seen before

POC: This video proofs its functionality both in desktop and mobile frontends

Summary by cubic

Adds bulk deletion of historical sessions from the sidebar with a simple selection mode and confirmation. Implements server/client support and i18n updates to address #2603.

• New Features
  • Backend: DELETE /project/threads to remove multiple threads by threadIds with auth and author checks.
  • Frontend: Select mode in ThreadList with checkboxes and bulk delete + confirm dialog and toasts; clears current/resumed thread when deleted; three-dot thread menu always visible on mobile.
  • API: Added ChainlitAPI.deleteThreads(threadIds).
  • i18n: Added “select” action and plural delete descriptions across locales.

^{Written for commit d3ba9fa. Summary will update on new commits.}