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.

app/controllers/snaptrade_items_controller.rb

@@ -154,6 +154,14 @@ class SnaptradeItemsController < ApplicationController
       @snaptrade_item.sync_later
     end
 
+    # Existing unlinked, visible investment/crypto accounts that could be linked instead of creating duplicates
+    @linkable_accounts = Current.family.accounts
+      .visible
+      .where(accountable_type: %w[Investment Crypto])
+      .left_joins(:account_providers)
+      .where(account_providers: { id: nil })
+      .order(:name)
+
     # Determine view state
     @syncing = @snaptrade_item.syncing?
     @waiting_for_sync = no_accounts && @syncing
@@ -369,9 +377,10 @@ class SnaptradeItemsController < ApplicationController
   def link_existing_account
     account_id = params[:account_id]
     snaptrade_account_id = params[:snaptrade_account_id]
+    snaptrade_item_id = params[:snaptrade_item_id]
 
     account = Current.family.accounts.find_by(id: account_id)
-    snaptrade_item = Current.family.snaptrade_items.first
+    snaptrade_item = Current.family.snaptrade_items.find_by(id: snaptrade_item_id)
     snaptrade_account = snaptrade_item&.snaptrade_accounts&.find_by(id: snaptrade_account_id)
 
     if account && snaptrade_account