* 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.
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