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.

migrations/011_clients_force_sync.sql

@@ -0,0 +1,2 @@
+ALTER TABLE `clients` ADD COLUMN `force_sync_campaigns` TINYINT(1) NOT NULL DEFAULT 0 AFTER `deleted`;
+ALTER TABLE `clients` ADD COLUMN `force_sync_products` TINYINT(1) NOT NULL DEFAULT 0 AFTER `force_sync_campaigns`;