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>

config.py

@@ -28,6 +28,7 @@ class Config:
 
     # Directories
     BACKGROUNDS_DIR = os.path.join('static', 'backgrounds')
+    PHOTOS_DIR = os.path.join('static', 'photos')
     CREDENTIALS_DIR = 'credentials'
 
     # Family members for event color-coding.