fix(diashow): working transitions + slide/push, fullscreen, activity controls

Transitions never actually animated: Svelte scopes @keyframes names but does NOT
rewrite animation references in inline style attributes, so the inline
'animation: crossfade-in ...' never matched its scoped keyframe — a hard cut, no fade
(Ken Burns' zoom was dead too). Move the animation into scoped classes and pass dynamic
values via CSS custom properties.

- Real two-layer crossfade: hold the outgoing frame opaque beneath the incoming one
  (which is decode-gated), so there's no black flash and no decode pop.
- New slide transitions (from below/left/right) as one reusable component; the previous
  frame is pushed off the opposite edge in step (a conveyor/push), easeInOutSine 1s.
- Fullscreen toggle button (Fullscreen API) + 'f' shortcut; Esc in fullscreen no longer
  also navigates away.
- Controls now reveal on pointer/keyboard activity and fade when idle (cursor hides too)
  instead of being always visible.
- wakelock: null the sentinel on the OS 'release' event so re-acquire-on-visible
  actually fires (previously the screen could sleep after the tab was first hidden).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/diashow/transitions/crossfade.svelte

@@ -14,9 +14,13 @@
 	let { src, isVideo, durationMs, onended }: Props = $props();
 </script>
 
+<!-- The fade lives in a scoped class (not an inline `style="animation:…"`): Svelte scopes
+	 `@keyframes` names and only rewrites `animation` references inside this <style> block —
+	 an inline animation name stays unscoped and silently never matches, so the fade would
+	 never run. The dynamic duration is passed through as a CSS custom property instead. -->
 <div
-	class="absolute inset-0 flex items-center justify-center bg-black"
-	style="animation: crossfade-in {durationMs}ms ease-out forwards;"
+	class="slide absolute inset-0 flex items-center justify-center bg-black"
+	style="--fade-dur: {durationMs}ms"
 >
 	{#if isVideo}
 		<!-- svelte-ignore a11y_media_has_caption -->
@@ -27,6 +31,9 @@
 </div>
 
 <style>
+	.slide {
+		animation: crossfade-in var(--fade-dur, 400ms) ease-out forwards;
+	}
 	@keyframes crossfade-in {
 		from {
 			opacity: 0;