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/helpers.js

@@ -40,11 +40,13 @@ function $(id) {
 function showError(id, msg) {
     const el = $(id);
     el.textContent = msg;
-    el.classList.remove("hidden");
+    el.style.visibility = "visible";
 }
 
 function hideError(id) {
-    $(id).classList.add("hidden");
+    const el = $(id);
+    el.textContent = "";
+    el.style.visibility = "hidden";
 }
 
 function showView(name) {