Fix slideshow crossfade flash — wait for image load before fading in

Previously the active class was applied via requestAnimationFrame
before the image had decoded, causing a brief blank-then-flash.
Now loadSlide waits for onload before transitioning, and advanceSlide
checks complete/naturalWidth (or waits for onload) on the preloaded
slot before doing the swap.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/js/app.js

@@ -302,10 +302,17 @@ function hideSlideshow() {
 
 function loadSlide(slot, url) {
     const img = document.getElementById(`slide-${slot}`);
-    img.src = url;
     if (slot === activeSlot) {
-        // Slight delay lets the browser decode before fading in
-        requestAnimationFrame(() => requestAnimationFrame(() => img.classList.add('active')));
+        // Only fade in once the image is fully decoded — no blank-then-flash
+        img.onload = () => { img.onload = null; img.classList.add('active'); };
+        img.src = url;
+        // Handle cached images that won't fire onload
+        if (img.complete && img.naturalWidth > 0) {
+            img.onload = null;
+            img.classList.add('active');
+        }
+    } else {
+        img.src = url; // preload only
     }
 }
 
@@ -314,13 +321,20 @@ function advanceSlide() {
     const current  = document.getElementById(`slide-${activeSlot}`);
     const next     = document.getElementById(`slide-${nextSlot}`);
 
-    // Fade in the preloaded next image, fade out current
+    const doSwap = () => {
+        next.onload = null;
         next.classList.add('active');
         current.classList.remove('active');
         activeSlot = nextSlot;
-
-    // Preload the one after next into the now-hidden slot
+        // Preload the upcoming photo into the now-hidden slot
         slideIndex = (slideIndex + 1) % photos.length;
-    const upcoming = photos[(slideIndex + 1) % photos.length];
-    current.src = upcoming;
+        current.src = photos[(slideIndex + 1) % photos.length];
+    };
+
+    // If the preloaded image is ready, swap immediately; otherwise wait for it
+    if (next.complete && next.naturalWidth > 0) {
+        doSwap();
+    } else {
+        next.onload = doSwap;
+    }
 }