fix(entries): toggle the secret flag in place instead of rebuilding every row

rerender() wipes the whole custom-fields list, which reads as the panel flashing and also drops focus and the auto-sized height of a multiline field.

js/app.js

@@ -4290,8 +4290,22 @@ function buildCustomFieldRow(field, idx, rerender) {
     secretBtn.addEventListener('click', ev => {
         ev.stopPropagation();
         field.is_secret = !field.is_secret;
+        // Update this row in place. rerender() rebuilds every row from scratch
+        // (list.innerHTML = ''), which the user sees as the panel flashing —
+        // and it would drop the focus and the auto-sized height too.
+        secretBtn.classList.toggle('is-on', field.is_secret);
+        secretBtn.textContent = field.is_secret ? '🔒' : '👁';
+        secretBtn.title = field.is_secret
+            ? 'Stored as secret — masked every time it is opened. Click to make it public.'
+            : 'Public field — shown in clear. Click to store it as a secret.';
+        if (field.multiline)
+            valueInput.classList.toggle('is-masked', field.is_secret);
+        else if (valueInput.tagName === 'INPUT')
+            valueInput.type = field.is_secret ? 'password' : 'text';
+        eye.style.visibility =
+            (!field.is_secret || (valueInput.tagName !== 'INPUT' && !field.multiline))
+                ? 'hidden' : '';
         soDirtyCheck();
-        rerender();
     });
 
     const copyBtn = el('button', { class: 'icon-btn icon-btn-sm', type: 'button',