Fix current_holdings to handle different dates and hide zero quantities

- Use DISTINCT ON to get most recent holding per security instead of assuming same date
- Filter out zero quantity holdings from UI display
- Maintain cash display regardless of zero balance
- Use single efficient query with proper Rails syntax

app/models/account.rb

@@ -82,7 +82,14 @@ class Account < ApplicationRecord
   end
 
   def current_holdings
-    holdings.where(currency: currency, date: holdings.maximum(:date)).order(amount: :desc)
+    holdings.where(currency: currency)
+            .where.not(qty: 0)
+            .where(
+              id: holdings.select("DISTINCT ON (security_id) id")
+                          .where(currency: currency)
+                          .order(:security_id, :date => :desc)
+            )
+            .order(amount: :desc)
   end
 
   def update_with_sync!(attributes)