fix: always use 'Transaction' as detail view heading

The transaction detail view was dynamically changing its title to match
the transaction type (e.g. 'Swap' for contract interactions), causing
inconsistency with the Screen Map specification. The heading is now
always 'Transaction' regardless of type. The action type is still
shown in the 'Action' detail section below.

Closes #65

src/popup/views/transactionDetail.js

@@ -143,11 +143,10 @@ function render() {
             typeEl.textContent = tx.directionLabel;
             typeSection.classList.remove("hidden");
         }
-        if (headingEl) headingEl.textContent = tx.directionLabel;
     } else {
         if (typeSection) typeSection.classList.add("hidden");
-        if (headingEl) headingEl.textContent = "Transaction";
     }
+    if (headingEl) headingEl.textContent = "Transaction";
 
     // Hide calldata and raw data sections; re-fetch if this is a contract call
     const calldataSection = $("tx-detail-calldata-section");