Confirm destructive folder deletes in chat explorer

From the PR description

Summary

• route assistant chat explorer folder deletes through a confirmation popup
• show how many nested folders and documents will be permanently deleted
• remove deleted documents from local chat explorer state and close stale document tabs after deletion

Bug

The assistant chat project explorer called the recursive folder-delete endpoint immediately. That backend endpoint permanently deletes documents in the folder tree, but this view updated local state as if those documents were only moved out of the folder. A user could delete documents from the chat explorer without a destructive-action warning and keep seeing stale document entries until refresh.

Verification

• NEXT_PUBLIC_SUPABASE_URL=http://localhost:54321 NEXT_PUBLIC_SUPABASE_PUBLISHABLE_DEFAULT_KEY=dummy npm run build --prefix frontend
• npm run build --prefix backend

Note: running the frontend build without placeholder Supabase public env values still hits the existing supabaseUrl is required prerender failure fixed separately in #173.