add "trade modified" warning if items are removed/reduced, configurable

Add config for requiring second confirmation screen, enabled by default
More resilience to adding items to trade after moving to confirmation screen

README.md

@@ -12,6 +12,7 @@ Players can request a trade, review both offers in a shared GUI, accept once to
 - Shared trade screen with separate offer areas for both players
 - Two-step confirmation flow
 - Quantity-based item selection with quick amounts and `Trade X`
+- Warning if items are removed/modified
 - Inventory-safe finalization: nothing changes until safety and security checks pass and the trade succeeds
 - Configurable trade safety checks to prevent dangerous mid-combat or mid-movement trading
 - Per-player trade toggles and ignore list support
@@ -49,6 +50,12 @@ They can also respond manually:
 - Once both players accept, the screen enters the confirmation stage
 - Click `Confirm` to finalize
 
+If a player reduces or removes items from an offer:
+
+- any pending accepts / confirms are invalidated
+- the changed offer slot gets a blinking red border and `!`
+- the trade window shows a red `Trade Modified` warning
+
 ## Commands
 
 ### Player commands
@@ -109,6 +116,7 @@ The trade verifies:
 - both players are still within configured trade distance, if enabled
 - both live inventories still match the snapshots taken when the trade started
 - both players can receive the incoming items
+- both players have re-accepted after any offer changes
 
 If any check fails, the trade is cancelled and both players receive an explicit chat message explaining why.
 
@@ -133,6 +141,16 @@ Server config values live under the `trade` section.
   - default: `false`
   - enables debug commands and debug UI/testing tools
 
+### Trade flow
+
+- `requireSecondConfirmation`
+  - default: `true`
+  - requires the second confirm step after both players accept the offer
+
+- `showTradeModifiedWarnings`
+  - default: `true`
+  - shows the `Trade Modified` warning and changed-slot highlights when offers are reduced or removed
+
 ### Safety
 
 - `requireOnGround`
@@ -196,4 +214,4 @@ Debug tools are disabled by default and only available when:
 
 - `enableDebugFeatures = true`
 
-When enabled, the mod exposes `/trade debug ...` commands and on-screen debug controls for single-client testing.
+When enabled, the mod exposes `/trade debug ...` commands and on-screen debug controls for single-client testing.