Simplify slideshow: load-on-demand, no preloading; add JS cache-busting

advanceSlide now sets src and waits for onload on the next slot directly —
no preloading, no transitionend, no setTimeout. Local files load in
milliseconds so preloading is unnecessary complexity that was causing
the slideshow to stall.

Also adds ?v=<mtime> cache-busting to the app.js script tag so Chromium
always picks up new JS after a deploy.

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

templates/index.html

@@ -32,6 +32,6 @@
         <div class="slideshow-hint">Touch to return to calendar</div>
     </div>
 
-    <script src="{{ url_for('static', filename='js/app.js') }}"></script>
+    <script src="{{ url_for('static', filename='js/app.js') }}?v={{ cache_bust }}"></script>
 </body>
 </html>