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/approval.js

@@ -269,7 +269,7 @@ function showTxApproval(details) {
     }
 
     $("approve-tx-password").value = "";
-    $("approve-tx-error").classList.add("hidden");
+    hideError("approve-tx-error");
 
     showView("approve-tx");
 }
@@ -351,10 +351,10 @@ function showSignApproval(details) {
     if (warningEl) {
         if (sp.dangerWarning) {
             warningEl.textContent = sp.dangerWarning;
-            warningEl.classList.remove("hidden");
+            warningEl.style.visibility = "visible";
         } else {
             warningEl.textContent = "";
-            warningEl.classList.add("hidden");
+            warningEl.style.visibility = "hidden";
         }
     }