feat: Implement campaign synchronization feature with dropdown UI

- Updated SCSS styles for new campaign sync buttons and dropdowns.
- Refactored main_view.php to replace the single select for campaigns with a multi-select dropdown.
- Added JavaScript functions to handle dropdown interactions and sync status updates.
- Introduced sync status bars for clients in main_view.php.
- Created new database migrations for client sync flags and cron sync status tracking.

.claude/settings.local.json

@@ -14,7 +14,8 @@
       "Bash(cd:*)",
       "Bash(git push:*)",
       "Bash(git add:*)",
-      "Bash(git commit:*)"
+      "Bash(git commit:*)",
+      "Bash(php:*)"
     ]
   }
 }