Add photo slideshow screensaver with inactivity timer

- After 1 hour of inactivity the screen switches to a fullscreen
  crossfade slideshow of photos from static/photos/
- Touch anywhere on screen to dismiss and return to the calendar
- /api/photos endpoint serves a shuffled list of images from static/photos/
- Two-slot A/B image swap for smooth 1.5s crossfades; next image preloads
  in the background while current is displayed (15s per photo)
- Slideshow only activates if static/photos/ contains at least one image

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

static/css/style.css

@@ -299,6 +299,45 @@ html, body {
     transform: translateY(0);
 }
 
+/* ---------- Photo slideshow screensaver ---------- */
+
+.slideshow-overlay {
+    position: fixed;
+    inset: 0;
+    background: #000;
+    z-index: 100;
+}
+
+.slideshow-overlay.hidden {
+    display: none;
+}
+
+.slide-img {
+    position: absolute;
+    inset: 0;
+    width: 100%;
+    height: 100%;
+    object-fit: cover;
+    opacity: 0;
+    transition: opacity 1.5s ease-in-out;
+}
+
+.slide-img.active {
+    opacity: 1;
+}
+
+.slideshow-hint {
+    position: absolute;
+    bottom: 32px;
+    left: 50%;
+    transform: translateX(-50%);
+    color: rgba(255, 255, 255, 0.35);
+    font-size: 13px;
+    letter-spacing: 0.04em;
+    pointer-events: none;
+    white-space: nowrap;
+}
+
 /* Scrollbar styling for day columns */
 .day-events::-webkit-scrollbar {
     width: 4px;