feat: add save-for-later buttons to frontend pages

Add shared save-later.js module and per-item save buttons to
blogroll, podroll, listening, and news pages. Buttons are hidden
by default and only visible when logged in. Posts to the readlater
plugin API at /readlater/save.

js/save-later.js

@@ -0,0 +1,51 @@
+/**
+ * Save for Later — shared frontend module
+ * Handles save button clicks on blogroll, podroll, listening, and news pages.
+ * Only active when user is logged in (body[data-indiekit-auth="true"]).
+ */
+(function () {
+  function isLoggedIn() {
+    return document.body.getAttribute('data-indiekit-auth') === 'true';
+  }
+
+  async function saveForLater(button) {
+    var url = button.dataset.saveUrl;
+    var title = button.dataset.saveTitle || url;
+    var source = button.dataset.saveSource || 'manual';
+    if (!url) return;
+
+    button.disabled = true;
+
+    try {
+      var response = await fetch('/readlater/save', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ url: url, title: title, source: source }),
+        credentials: 'same-origin'
+      });
+
+      if (response.ok) {
+        button.classList.add('save-later--saved');
+        button.title = 'Saved';
+        button.setAttribute('aria-label', 'Saved');
+        var label = button.querySelector('.save-later-label');
+        if (label) label.textContent = 'Saved';
+        var icon = button.querySelector('.save-later-icon');
+        if (icon) icon.textContent = '🔖';
+      } else {
+        button.disabled = false;
+      }
+    } catch (e) {
+      button.disabled = false;
+    }
+  }
+
+  document.addEventListener('click', function (e) {
+    if (!isLoggedIn()) return;
+    var button = e.target.closest('.save-later-btn');
+    if (button) {
+      e.preventDefault();
+      saveForLater(button);
+    }
+  });
+})();