Add transaction fee support to trades (#1248)

Add an optional fee field (decimal, precision: 19, scale: 4) to trades.
Fee is included in the total amount calculation (qty * price + fee) for
both create and update flows. The fee field appears on both the create
and edit forms, defaults to 0, and auto-submits like other trade fields.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

db/migrate/20260322120702_add_fee_to_trades.rb

@@ -0,0 +1,5 @@
+class AddFeeToTrades < ActiveRecord::Migration[7.2]
+  def change
+    add_column :trades, :fee, :decimal, precision: 19, scale: 4, default: 0, null: false
+  end
+end