fix(dashboard): current spread sign for HL→BG positions

Bug: current spread always computed as (hl-bg)/bg regardless of
position direction. For HL→BG positions, spread should be (bg-hl)/hl
to match entry spread convention.

Fix: check LongLeg.Exchange — if HL→BG, use (bg-hl)/hl instead.

web/static/app.js

@@ -603,6 +603,10 @@ document.addEventListener('keydown', function(e) {
   if (e.key === 'Escape') closeTradeDetail();
 });
 
+// Expose modal functions to global scope for HTML onclick handlers
+window.openTradeDetail = openTradeDetail;
+window.closeTradeDetail = closeTradeDetail;
+
 // ---- Init ----
 function init() {
   connectSSE();