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/addWallet.js

@@ -71,7 +71,7 @@ function show() {
     $("import-xprv-key").value = "";
     $("add-wallet-password").value = "";
     $("add-wallet-password-confirm").value = "";
-    $("add-wallet-phrase-warning").classList.add("hidden");
+    $("add-wallet-phrase-warning").style.visibility = "hidden";
     switchMode("mnemonic");
     showView("add-wallet");
 }
@@ -281,7 +281,7 @@ function init(ctx) {
     // Generate mnemonic
     $("btn-generate-phrase").addEventListener("click", () => {
         $("wallet-mnemonic").value = generateMnemonic();
-        $("add-wallet-phrase-warning").classList.remove("hidden");
+        $("add-wallet-phrase-warning").style.visibility = "visible";
     });
 
     // Import / confirm