chore: remove dead folders and untrack generated build artifacts

- Move dead firebase_twitter_sign_in (no pubspec), vendored
  flutter_date_pickers-master, and temp .pr-description dirs to .trash
- Untrack 75 .dart_tool files and a stray pubspec.lock from git
- Consolidate stray in-app .trash dirs; ignore .pr-description-*

.gitignore

@@ -351,5 +351,6 @@ build/
 
 **/flutter_export_environment.sh
 **/.dart_tool
+.pr-description-*/
 
 .trash/