Improve handling of cost_basis during holding materialization and display (#619)

- Refactored `persist_holdings` to separate and conditionally upsert holdings with and without cost_basis.
- Updated `avg_cost` logic to treat 0 cost_basis as unknown and return nil when cost_basis cannot be determined.
- Modified trend and investment calculation to exclude holdings with unknown cost_basis.
- Adjusted `average_cost` formatting to handle nil values in API responses and views.
- Added comprehensive tests to ensure cost_basis preservation and fallback behavior.
- Localized `unknown` label for display when cost_basis is unavailable.

Co-authored-by: Josh Waldrep <joshua.waldrep5+github@gmail.com>

app/models/investment_statement.rb

@@ -133,8 +133,12 @@ class InvestmentStatement
     holdings = current_holdings.to_a
     return nil if holdings.empty?
 
-    current = holdings.sum(&:amount)
-    previous = holdings.sum { |h| h.qty * h.avg_cost.amount }
+    # Only include holdings with known cost basis in the calculation
+    holdings_with_cost_basis = holdings.select(&:avg_cost)
+    return nil if holdings_with_cost_basis.empty?
+
+    current = holdings_with_cost_basis.sum(&:amount)
+    previous = holdings_with_cost_basis.sum { |h| h.qty * h.avg_cost.amount }
 
     Trend.new(current: current, previous: previous)
   end