feat(diashow): live slideshow with two-queue policy + pluggable transitions

A fullscreen auto-advancing slideshow any user can start. Design:
docs/CONCEPT_DIASHOW.md.

- lib/diashow/queue.ts: SlideQueue state machine — liveQueue drains first
  (FIFO, seeded by SSE upload-processed), then shuffleQueue (refilled
  from allKnown minus a 5-id ring buffer of recently shown). Pure logic,
  unit-testable.
- lib/diashow/wakelock.ts: Screen Wake Lock wrapper that re-acquires on
  visibility change (the OS drops the lock when the tab hides).
- lib/diashow/transitions/{index,crossfade,kenburns}.ts: registry +
  the v1 transitions. Adding a new animation is one file + one entry —
  the extensibility target from docs/FEATURES §2.9.
- routes/diashow/+page.svelte: fullscreen page, hides bottom nav,
  6 s default dwell (3/6/10 configurable), keyboard shortcuts
  (Escape exits, Space toggles pause), tap-to-reveal overlay with
  pause / dwell / transition / exit. Respects $dataMode to choose
  preview vs. original URL.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,38 @@
+<script lang="ts">
+	// Simplest transition: render whatever the parent gave us, full-bleed, with a CSS
+	// opacity fade. Tied to the parent's `{#key}` block — when the src changes, the
+	// parent re-mounts this component and the `in:` transition runs.
+
+	interface Props {
+		src: string;
+		isVideo: boolean;
+		durationMs: number;
+	}
+
+	let { src, isVideo, durationMs }: Props = $props();
+</script>
+
+<div
+	class="absolute inset-0 flex items-center justify-center bg-black"
+	style="animation: crossfade-in {durationMs}ms ease-out forwards;"
+>
+	{#if isVideo}
+		<!-- svelte-ignore a11y_media_has_caption -->
+		<video
+			{src}
+			autoplay
+			muted
+			playsinline
+			class="h-full w-full object-contain"
+		></video>
+	{:else}
+		<img {src} alt="" class="h-full w-full object-contain" />
+	{/if}
+</div>
+
+<style>
+	@keyframes crossfade-in {
+		from { opacity: 0; }
+		to   { opacity: 1; }
+	}
+</style>