Fix UX mismatch and dead code in paper trading

- Hide exchange dropdown for paper trades (uses single synthetic market)
- Add _updateExchangeRowVisibility() to toggle exchange row visibility
- Call visibility update on target change and form open
- Remove stale 'binance' fallback in Trade constructor call
  (Trade.__init__ now handles exchange normalization for paper trades)

Addresses codex feedback:
- Medium: UX mismatch where exchange choice appeared relevant for paper
- Low: Dead code passing 'binance' for paper trades

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/static/trade.js

@@ -19,6 +19,7 @@ class TradeUIManager {
         this.stopLossInput = null;
         this.takeProfitInput = null;
         this.timeInForceSelect = null;
+        this.exchangeRow = null;
         this.onCloseTrade = null;
 
         // Exchanges known to support testnet/sandbox mode
@@ -56,7 +57,8 @@ class TradeUIManager {
             testnetRowId = 'testnet-row',
             stopLossId = 'stopLoss',
             takeProfitId = 'takeProfit',
-            timeInForceId = 'timeInForce'
+            timeInForceId = 'timeInForce',
+            exchangeRowId = 'exchange-row'
         } = config;
 
         this.targetEl = document.getElementById(targetId);
@@ -83,6 +85,7 @@ class TradeUIManager {
         this.stopLossInput = document.getElementById(stopLossId);
         this.takeProfitInput = document.getElementById(takeProfitId);
         this.timeInForceSelect = document.getElementById(timeInForceId);
+        this.exchangeRow = document.getElementById(exchangeRowId);
 
         // Set up event listeners
         this._setupFormListeners();
@@ -129,10 +132,11 @@ class TradeUIManager {
             this.qtyInput.addEventListener('input', updateTradeValue);
         }
 
-        // Trade target (exchange) changes affect testnet visibility and SELL availability
+        // Trade target (exchange) changes affect testnet visibility, exchange row, and SELL availability
         if (this.targetSelect) {
             this.targetSelect.addEventListener('change', () => {
                 this._updateTestnetVisibility();
+                this._updateExchangeRowVisibility();
                 this._updateSellAvailability();
             });
         }
@@ -298,6 +302,25 @@ class TradeUIManager {
         }
     }
 
+    /**
+     * Updates exchange row visibility based on trade mode.
+     * Paper trades use a single synthetic market, so exchange selection is irrelevant.
+     */
+    _updateExchangeRowVisibility() {
+        if (!this.exchangeRow || !this.targetSelect) return;
+
+        const selectedTarget = this.targetSelect.value;
+        const isPaperTrade = selectedTarget === 'test_exchange';
+
+        if (isPaperTrade) {
+            // Hide exchange row for paper trading (uses single synthetic market)
+            this.exchangeRow.style.display = 'none';
+        } else {
+            // Show exchange row for live exchanges
+            this.exchangeRow.style.display = 'contents';
+        }
+    }
+
     /**
      * Populates the exchange selector with connected exchanges.
      * @param {string[]} connectedExchanges - List of connected exchange names.
@@ -404,13 +427,17 @@ class TradeUIManager {
         const exchangeToUse = chartExchange || 'binance';
         await this._populateSymbolDropdown(exchangeToUse, symbol);
 
-        // Reset to paper trade and hide testnet row
+        // Reset to paper trade and hide testnet/exchange rows
         if (this.targetSelect) {
             this.targetSelect.value = 'test_exchange';
         }
         if (this.testnetRow) {
             this.testnetRow.style.display = 'none';
         }
+        if (this.exchangeRow) {
+            // Hide exchange row for paper trading (uses single synthetic market)
+            this.exchangeRow.style.display = 'none';
+        }
         if (this.testnetCheckbox) {
             this.testnetCheckbox.checked = true;
         }

src/templates/new_trade_popup.html

@@ -31,11 +31,13 @@
                 </small>
             </div>
 
-            <!-- Exchange Selection -->
-            <label for="tradeExchange"><b>Exchange:</b></label>
-            <select name="tradeExchange" id="tradeExchange">
-                <!-- Exchanges populated dynamically, defaults to chart view -->
-            </select>
+            <!-- Exchange Selection (for price data source - hidden for paper trades) -->
+            <div id="exchange-row" style="display: contents;">
+                <label for="tradeExchange"><b>Exchange:</b></label>
+                <select name="tradeExchange" id="tradeExchange">
+                    <!-- Exchanges populated dynamically, defaults to chart view -->
+                </select>
+            </div>
 
             <!-- Symbol Selection -->
             <label for="tradeSymbol"><b>Symbol:</b></label>

src/trade.py

@@ -685,9 +685,10 @@ class Trades:
                 trade_status = status_map.get(result.status, 'pending')
 
                 # Create Trade with full broker tracking
+                # Note: Trade.__init__ normalizes exchange to 'paper' for paper trades
                 trade = Trade(
                     target=target,
-                    exchange=exchange or (target if not is_paper else 'binance'),
+                    exchange=exchange,
                     symbol=symbol,
                     side=side.upper(),
                     order_price=effective_price,