Fix slideshow mid-transition flash — delay src change until fade-out completes

Changing current.src immediately triggered the new image to render while
the element was still mid-opacity-transition (partially visible), causing
the next queued photo to briefly flash through the crossfade. Now we wait
for the transitionend event before setting the new preload src.

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

static/js/app.js

@@ -326,9 +326,11 @@ function advanceSlide() {
         next.classList.add('active');
         current.classList.remove('active');
         activeSlot = nextSlot;
-        // Preload the upcoming photo into the now-hidden slot
         slideIndex = (slideIndex + 1) % photos.length;
-        current.src = photos[(slideIndex + 1) % photos.length];
+        const preloadSrc = photos[(slideIndex + 1) % photos.length];
+        // Only change src after the fade-out completes — changing it mid-transition
+        // causes the new image to appear at partial opacity (the flash the user sees)
+        current.addEventListener('transitionend', () => { current.src = preloadSrc; }, { once: true });
     };
 
     // If the preloaded image is ready, swap immediately; otherwise wait for it