fix: reserve space for all error/status messages — closes #123

Replace display:none (hidden class) with visibility:hidden/visible for all
error, warning, and status message elements across the extension UI. This
prevents layout shift when messages appear or disappear.

Changes:
- helpers.js: showError/hideError now use visibility instead of hidden class
- index.html: all error/status divs use visibility:hidden + min-height
- confirmTx.js: warnings, errors, fee section use visibility
- approval.js: tx-error, sign-error, danger-warning use visibility
- addressDetail.js: export-privkey-flash uses visibility
- deleteWallet.js: delete-wallet-flash uses visibility
- addWallet.js: phrase-warning uses visibility
- receive.js: erc20-warning uses visibility
- addToken.js: add-token-info uses visibility
- settingsAddToken.js: settings-addtoken-info uses visibility

src/popup/views/confirmTx.js

@@ -186,9 +186,10 @@ function show(txInfo) {
                     `<div class="border border-border border-dashed p-2 mb-1 text-xs font-bold">WARNING: ${w}</div>`,
             )
             .join("");
-        warningsEl.classList.remove("hidden");
+        warningsEl.style.visibility = "visible";
     } else {
-        warningsEl.classList.add("hidden");
+        warningsEl.innerHTML = "";
+        warningsEl.style.visibility = "hidden";
     }
 
     // Check for errors
@@ -226,11 +227,12 @@ function show(txInfo) {
         errorsEl.innerHTML = errors
             .map((e) => `<div class="text-xs">${e}</div>`)
             .join("");
-        errorsEl.classList.remove("hidden");
+        errorsEl.style.visibility = "visible";
         sendBtn.disabled = true;
         sendBtn.classList.add("text-muted");
     } else {
-        errorsEl.classList.add("hidden");
+        errorsEl.innerHTML = "";
+        errorsEl.style.visibility = "hidden";
         sendBtn.disabled = false;
         sendBtn.classList.remove("text-muted");
     }
@@ -240,7 +242,7 @@ function show(txInfo) {
     hideError("confirm-tx-password-error");
 
     // Gas estimate — show placeholder then fetch async
-    $("confirm-fee").classList.remove("hidden");
+    $("confirm-fee").style.visibility = "visible";
     $("confirm-fee-amount").textContent = "Estimating...";
     state.viewData = { pendingTx: txInfo };
     showView("confirm-tx");