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/deleteWallet.js

@@ -12,7 +12,7 @@ function show(walletIdx) {
         wallet.name || "Wallet " + (walletIdx + 1);
     $("delete-wallet-password").value = "";
     $("delete-wallet-flash").textContent = "";
-    $("delete-wallet-flash").classList.add("hidden");
+    $("delete-wallet-flash").style.visibility = "hidden";
     showView("delete-wallet-confirm");
 }
 
@@ -29,14 +29,14 @@ function init(_ctx) {
         if (!pw) {
             $("delete-wallet-flash").textContent =
                 "Please enter your password.";
-            $("delete-wallet-flash").classList.remove("hidden");
+            $("delete-wallet-flash").style.visibility = "visible";
             return;
         }
 
         if (deleteWalletIndex === null) {
             $("delete-wallet-flash").textContent =
                 "No wallet selected for deletion.";
-            $("delete-wallet-flash").classList.remove("hidden");
+            $("delete-wallet-flash").style.visibility = "visible";
             return;
         }
 
@@ -52,7 +52,7 @@ function init(_ctx) {
             await decryptWithPassword(wallet.encryptedSecret, pw);
         } catch (_e) {
             $("delete-wallet-flash").textContent = "Wrong password.";
-            $("delete-wallet-flash").classList.remove("hidden");
+            $("delete-wallet-flash").style.visibility = "visible";
             btn.disabled = false;
             btn.classList.remove("text-muted");
             return;