fix: never collapse warning container — always reserve space to prevent layout shift

Replace display:none with persistent visibility:hidden so the warning
area occupies the same vertical space regardless of API result.
This eliminates the layout shift that occurred when the container was
collapsed after the recipient history check returned.

src/popup/views/confirmTx.js

@@ -246,7 +246,6 @@ function show(txInfo) {
     // Reset recipient warning: reserve space (visibility:hidden) while
     // the async check runs, preventing layout shift per README policy.
     const recipientWarning = $("confirm-recipient-warning");
-    recipientWarning.style.display = "";
     recipientWarning.style.visibility = "hidden";
 
     estimateGas(txInfo);
@@ -302,22 +301,18 @@ async function checkRecipientHistory(txInfo) {
         // the nonce, i.e. sent-tx count only).
         const code = await provider.getCode(txInfo.to);
         if (code && code !== "0x") {
-            // Contract address — hide the reserved space entirely
-            el.style.display = "none";
+            // Contract address — no warning needed, keep space reserved
+            // but invisible to prevent layout shift
             return;
         }
         const txCount = await provider.getTransactionCount(txInfo.to);
         if (txCount === 0) {
             el.style.visibility = "visible";
-        } else {
-            // Address has history — collapse the reserved space
-            el.style.display = "none";
         }
+        // If txCount > 0, leave visibility:hidden — space stays reserved
     } catch (e) {
         log.errorf("recipient history check failed:", e.message);
-        // On error, collapse the reserved space rather than showing a
-        // false warning or leaving an empty gap
-        el.style.display = "none";
+        // On error, leave visibility:hidden — no layout shift, no false warning
     }
 }