fix(dashboard): make auth-image hydration lazy by default (#182 follow-up) (#185)

#182 shipped hydrateAuthImages() loading every thumbnail immediately, which
regressed the content-library grid from lazy to eager (a fetch per thumbnail on
render) and left the IntersectionObserver as dead code.

Restore lazy-by-default (observe-only) so large grids only fetch thumbnails as
they scroll into view, and add an { eager: true } opt-in for the transient
pickers where every item is on screen and immediate load reads better: the
device assign-content modal, the playlist add-item modal, and the widget
content picker. Grids and inline lists (content library, playlist items, device
playlist tab, directory logo/background) use the lazy default.

Behavior for those pickers is unchanged; only the large grids revert to the
lazy loading they had before #182.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

frontend/js/utils.js

@@ -73,8 +73,8 @@ export function isPlatformAdmin(user) {
 
 // Lazy-load authenticated images. A plain <img> can't send the Bearer token,
 // and thumbnail/file endpoints require auth — a just-uploaded item's thumbnail
-// 403's without it. We fetch with the token and swap in an object URL.
-// IntersectionObserver keeps it lazy; the object URL is revoked after load.
+// 403's without it. We fetch with the token and swap in an object URL, revoked
+// after load.
 let _authImgObserver = null;
 export function loadAuthImage(img) {
   const url = img.dataset.authSrc;
@@ -89,13 +89,17 @@ export function loadAuthImage(img) {
     })
     .catch(() => { img.style.opacity = '0.25'; });
 }
-export function hydrateAuthImages(root) {
+// Hydrate <img data-auth-src> under `root`. Lazy by default: an
+// IntersectionObserver loads each thumbnail only as it scrolls into view, so a
+// large grid (content library, etc.) doesn't fire a fetch per thumbnail on
+// render. Pass { eager: true } for small, transient surfaces (pickers/modals)
+// where every item is on screen and immediate load reads better.
+export function hydrateAuthImages(root, { eager = false } = {}) {
   const imgs = root.querySelectorAll('img[data-auth-src]');
   if (!imgs.length) return;
 
-  // Load all images immediately; IntersectionObserver is used below
-  // only for images that are off-screen (lazy loading).
-  if (typeof IntersectionObserver === 'undefined') {
+  // Eager path (opt-in) and the no-IntersectionObserver fallback both load now.
+  if (eager || typeof IntersectionObserver === 'undefined') {
     imgs.forEach(loadAuthImage);
     return;
   }
@@ -105,8 +109,5 @@ export function hydrateAuthImages(root) {
       for (const e of entries) if (e.isIntersecting) { obs.unobserve(e.target); loadAuthImage(e.target); }
     }, { rootMargin: '300px' });
   }
-
-  // Load every image now — the observer will also fire for them but
-  // loadAuthImage is idempotent (deletes data-auth-src on first call).
-  imgs.forEach(img => { loadAuthImage(img); _authImgObserver.observe(img); });
+  imgs.forEach(img => _authImgObserver.observe(img));
 }

frontend/js/views/device-detail.js

@@ -1454,7 +1454,7 @@ async function setupPlaylistActions(device) {
         </div>
       `;
       document.body.appendChild(modal);
-      hydrateAuthImages(modal);
+      hydrateAuthImages(modal, { eager: true });
 
       // Tab switching
       modal.querySelectorAll('.assign-tab').forEach(tab => {

frontend/js/views/playlists.js

@@ -687,7 +687,7 @@ async function showAddItemModal(playlistId, opts = {}) {
         </div>
       `;
     }).join('');
-    hydrateAuthImages(list);
+    hydrateAuthImages(list, { eager: true });
 
     list.querySelectorAll('.add-item-btn').forEach(btn => {
       btn.addEventListener('click', async (e) => {

frontend/js/views/widgets.js

@@ -74,7 +74,7 @@ function openContentPicker({ multiple = false, title } = {}) {
             </div>`;
         }).join('')
       }</div>`;
-      hydrateAuthImages(list);
+      hydrateAuthImages(list, { eager: true });
       list.querySelectorAll('[data-pick-id]').forEach(el => el.onclick = () => {
         const id = el.dataset.pickId;
         if (multiple) {