Instant slide navigation and flicker-free page switches

Presenting a deck is now smooth: moving between slides no longer replaces the page with a progress bar and loading spinners. The portal keeps the current slide on screen, prepares the next one in the background, and swaps only when it is ready — so clicking through a presentation feels instant, and click-to-advance keeps working even while a slide is loading.

Behind the scenes, page navigation is served from a cache: revisiting a page resolves immediately instead of asking the server again, and progress indicators appear only when a navigation is genuinely slow. Slide decks also render correctly from the very first frame — the slide counter and Previous/Next controls show the right position immediately instead of briefly reading "Slide 1 / 1".