persist confirm-tx view across popup close/reopen (closes #77)

Add confirm-tx to RESTORABLE_VIEWS and save pendingTx in
state.viewData so the confirmation screen survives the popup
lifecycle. On restore, re-render the full confirmation view
including gas estimate.

src/popup/index.js

@@ -74,6 +74,7 @@ const RESTORABLE_VIEWS = new Set([
     "receive",
     "settings",
     "settings-addtoken",
+    "confirm-tx",
     "transaction",
     "success-tx",
     "error-tx",
@@ -127,6 +128,13 @@ function restoreView() {
         case "settings-addtoken":
             settingsAddToken.show();
             break;
+        case "confirm-tx":
+            if (state.viewData && state.viewData.pendingTx) {
+                confirmTx.restore();
+            } else {
+                fallbackView();
+            }
+            break;
         case "transaction":
             if (state.viewData && state.viewData.tx) {
                 transactionDetail.render();