fix: add fallback popup window for tx and sign approval requests (closes #4)

requestTxApproval and requestSignApproval were missing the
openApprovalWindow fallback that requestApproval (connection
requests) already had. When chrome.action.openPopup() fails
(which it does when not triggered by a user gesture in MV3),
no popup appeared — the user had to manually click the toolbar
icon to find the confirmation screen.

Now all three approval types use the same pattern: try
openPopup() first, fall back to a centered standalone window.

src/popup/index.js

@@ -189,7 +189,7 @@ async function init() {
     const params = new URLSearchParams(window.location.search);
     const approvalId = params.get("approval");
     if (approvalId) {
-        approval.show(parseInt(approvalId, 10));
+        approval.show(approvalId);
         showView("approve-site");
         return;
     }