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/addToken.js

@@ -7,7 +7,8 @@ const { log } = require("../../shared/log");
 
 function show() {
     $("add-token-address").value = "";
-    $("add-token-info").classList.add("hidden");
+    $("add-token-info").textContent = "";
+    $("add-token-info").style.visibility = "hidden";
     const list = $("common-token-list");
     list.innerHTML = getTopTokens(25)
         .map(
@@ -45,7 +46,7 @@ function init(ctx) {
         }
         const infoEl = $("add-token-info");
         infoEl.textContent = "Looking up token...";
-        infoEl.classList.remove("hidden");
+        infoEl.style.visibility = "visible";
         log.debugf("Looking up token contract", contractAddr);
         try {
             const info = await lookupTokenInfo(contractAddr, state.rpcUrl);
@@ -63,7 +64,8 @@ function init(ctx) {
             const detail = e.shortMessage || e.message || String(e);
             log.errorf("Token lookup failed for", contractAddr, detail);
             showFlash(detail);
-            infoEl.classList.add("hidden");
+            infoEl.textContent = "";
+            infoEl.style.visibility = "hidden";
         }
     });