* Add account linking functionality for SnapTrade items
- Introduced UI to link existing accounts when setting up SnapTrade items, preventing duplicate account creation.
- Updated controller to fetch linkable accounts.
- Added tests to verify proper filtering of accounts and linking behavior.
* Add `snaptrade_item_id` to account linking flow for SnapTrade items
- Updated controller to allow specifying `snaptrade_item_id` when linking accounts.
- Adjusted form and views to include `snaptrade_item_id` as a hidden field.
- Enhanced tests to validate behavior with the new parameter.
- Introduced new tests to cover SnapTrade decryption and connection errors in `SnaptradeItemsControllerTest`.
- Updated error messages for improved user clarity.
- Modified `unlink` functionality to preserve `SnaptradeAccount` records while ensuring proper detachment of associated holdings.
