Add "Link to existing" option in SnapTrade Setup Accounts modal (#935)

* 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.

config/locales/views/snaptrade_items/en.yml

@@ -75,6 +75,9 @@ en:
       cancel_button: "Cancel"
       creating: "Creating Accounts..."
       done_button: "Done"
+      or_link_existing: "Or link to an existing account instead of creating a new one:"
+      select_account: "Select an account..."
+      link_button: "Link"
       linked_accounts: "Already Linked"
       linked_to: "Linked to:"
     snaptrade_item: