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/queue.ts

@@ -0,0 +1,106 @@
+// Two-queue slide state machine. Pure logic — no DOM, no Svelte, no network. Lets us
+// unit-test the policy without spinning up a browser.
+//
+// Policy: live posts always drain first; the shuffle queue refills from `allKnown`
+// (minus the most recent N items) once it empties. A new live post pushes onto the
+// live queue and waits for the next slide transition — it never interrupts the
+// current slide.
+
+import type { FeedUpload } from '$lib/types';
+
+const RECENT_RING_SIZE = 5;
+
+export class SlideQueue {
+	/** Live queue — FIFO. New uploads land here via `pushLive`. */
+	private liveQueue: FeedUpload[] = [];
+	/** Shuffle queue — refilled from `allKnown` minus `recentlyShown` when emptied. */
+	private shuffleQueue: FeedUpload[] = [];
+	/** Every slide we've ever seen, keyed by id. Source for shuffle refills. */
+	private allKnown: Map<string, FeedUpload> = new Map();
+	/** Ring buffer of the last N shown ids — excluded from the next shuffle pool. */
+	private recentlyShown: string[] = [];
+
+	/** Seed the shuffle pool from an initial fetch of the feed. */
+	seed(initial: FeedUpload[]): void {
+		for (const slide of initial) {
+			this.allKnown.set(slide.id, slide);
+		}
+		this.shuffleQueue = shuffle(Array.from(this.allKnown.values()));
+	}
+
+	/** Add a slide pushed by a new SSE upload-processed event. */
+	pushLive(slide: FeedUpload): void {
+		if (this.allKnown.has(slide.id)) return;
+		this.allKnown.set(slide.id, slide);
+		this.liveQueue.push(slide);
+	}
+
+	/** Pop the next slide. Returns null while both queues are empty (event has no posts). */
+	next(): FeedUpload | null {
+		// 1. Drain live first.
+		const live = this.liveQueue.shift();
+		if (live) {
+			this.markShown(live.id);
+			return live;
+		}
+		// 2. Refill shuffle queue from `allKnown` minus recently shown.
+		if (this.shuffleQueue.length === 0) {
+			this.shuffleQueue = shuffle(
+				Array.from(this.allKnown.values()).filter(
+					(s) => !this.recentlyShown.includes(s.id)
+				)
+			);
+			// If everything is recently shown (small event), fall back to the full pool.
+			if (this.shuffleQueue.length === 0) {
+				this.shuffleQueue = shuffle(Array.from(this.allKnown.values()));
+			}
+		}
+		const next = this.shuffleQueue.shift() ?? null;
+		if (next) this.markShown(next.id);
+		return next;
+	}
+
+	/**
+	 * Remove a slide that was deleted or hidden. Returns true if it was the current
+	 * "head" and the caller should advance immediately (UX: don't keep showing a
+	 * post that the host just deleted).
+	 */
+	remove(id: string, currentId: string | null): { wasCurrent: boolean } {
+		this.allKnown.delete(id);
+		this.liveQueue = this.liveQueue.filter((s) => s.id !== id);
+		this.shuffleQueue = this.shuffleQueue.filter((s) => s.id !== id);
+		this.recentlyShown = this.recentlyShown.filter((sid) => sid !== id);
+		return { wasCurrent: currentId === id };
+	}
+
+	/** Look up a slide by id — for the diashow page to render the current slide. */
+	get(id: string): FeedUpload | undefined {
+		return this.allKnown.get(id);
+	}
+
+	/** Snapshot — useful for stats / debugging. */
+	stats() {
+		return {
+			known: this.allKnown.size,
+			live: this.liveQueue.length,
+			shuffle: this.shuffleQueue.length
+		};
+	}
+
+	private markShown(id: string): void {
+		this.recentlyShown.push(id);
+		while (this.recentlyShown.length > RECENT_RING_SIZE) {
+			this.recentlyShown.shift();
+		}
+	}
+}
+
+/** Fisher–Yates. Mutates a copy of the input so callers can pass `allKnown.values()`. */
+function shuffle<T>(arr: T[]): T[] {
+	const out = arr.slice();
+	for (let i = out.length - 1; i > 0; i--) {
+		const j = Math.floor(Math.random() * (i + 1));
+		[out[i], out[j]] = [out[j], out[i]];
+	}
+	return out;
+}

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>

frontend/src/lib/diashow/transitions/index.ts

@@ -0,0 +1,47 @@
+// Pluggable transition registry. The diashow consults this list to populate its
+// settings popover and to render the current transition.
+//
+// Adding a new animation:
+//   1. Drop a Svelte file alongside crossfade.svelte / kenburns.svelte.
+//   2. Add one entry to `transitions` below.
+//   3. Rebuild — the popover updates automatically.
+//
+// This is the extensibility principle from docs/FEATURES.md §2.9 made concrete:
+// no diashow code needs to change to add an animation.
+
+import type { Component } from 'svelte';
+import Crossfade from './crossfade.svelte';
+import KenBurns from './kenburns.svelte';
+
+export interface SlideTransition {
+	id: string;
+	label: string;
+	defaultDurationMs: number;
+	component: Component<TransitionProps>;
+}
+
+/** Props every transition Svelte component receives. */
+export interface TransitionProps {
+	src: string;
+	isVideo: boolean;
+	durationMs: number;
+}
+
+export const transitions: SlideTransition[] = [
+	{
+		id: 'crossfade',
+		label: 'Überblendung',
+		defaultDurationMs: 400,
+		component: Crossfade as unknown as Component<TransitionProps>
+	},
+	{
+		id: 'kenburns',
+		label: 'Ken Burns',
+		defaultDurationMs: 600,
+		component: KenBurns as unknown as Component<TransitionProps>
+	}
+];
+
+export function findTransition(id: string): SlideTransition {
+	return transitions.find((t) => t.id === id) ?? transitions[0];
+}

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

@@ -0,0 +1,51 @@
+<script lang="ts">
+	// Crossfade + slow zoom and pan (the "Ken Burns" effect). Image-only — videos
+	// fall back to the crossfade behaviour to avoid double-animation.
+
+	interface Props {
+		src: string;
+		isVideo: boolean;
+		durationMs: number;
+	}
+
+	let { src, isVideo, durationMs }: Props = $props();
+
+	// Mild random pan so each slide feels different. Range chosen so the image never
+	// pans out of frame given the object-fit: cover.
+	const panX = Math.round((Math.random() - 0.5) * 6); // -3% .. +3%
+	const panY = Math.round((Math.random() - 0.5) * 6);
+</script>
+
+<div
+	class="absolute inset-0 flex items-center justify-center overflow-hidden bg-black"
+	style="animation: kb-fade {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-cover"
+			style="animation: kb-zoom 10s ease-out forwards; transform-origin: {50 + panX}% {50 + panY}%;"
+		/>
+	{/if}
+</div>
+
+<style>
+	@keyframes kb-fade {
+		from { opacity: 0; }
+		to   { opacity: 1; }
+	}
+	@keyframes kb-zoom {
+		from { transform: scale(1.0); }
+		to   { transform: scale(1.1); }
+	}
+</style>

frontend/src/lib/diashow/wakelock.ts

@@ -0,0 +1,54 @@
+// Thin wrapper around the Screen Wake Lock API. Held by the diashow page so a phone
+// driving a projector doesn't sleep. No-op on unsupported browsers (Firefox, older
+// Safari).
+//
+// Wake locks die when the document goes hidden; the page re-acquires on visible to
+// keep the screen on across short interruptions.
+
+interface SentinelLike {
+	release: () => Promise<void>;
+}
+
+let sentinel: SentinelLike | null = null;
+let visibilityHandler: (() => void) | null = null;
+
+export async function acquireWakeLock(): Promise<void> {
+	const wakeLock = (navigator as Navigator & { wakeLock?: { request: (t: string) => Promise<SentinelLike> } }).wakeLock;
+	if (!wakeLock) return;
+	try {
+		sentinel = await wakeLock.request('screen');
+	} catch {
+		// User denied, or already released — nothing useful to do.
+		sentinel = null;
+	}
+
+	// Re-acquire when the page becomes visible again (the OS releases the lock
+	// while the tab is hidden).
+	if (!visibilityHandler) {
+		visibilityHandler = async () => {
+			if (document.visibilityState === 'visible' && sentinel === null) {
+				try {
+					sentinel = await wakeLock.request('screen');
+				} catch {
+					sentinel = null;
+				}
+			}
+		};
+		document.addEventListener('visibilitychange', visibilityHandler);
+	}
+}
+
+export async function releaseWakeLock(): Promise<void> {
+	if (sentinel) {
+		try {
+			await sentinel.release();
+		} catch {
+			// ignore — release after release is fine
+		}
+		sentinel = null;
+	}
+	if (visibilityHandler) {
+		document.removeEventListener('visibilitychange', visibilityHandler);
+		visibilityHandler = null;
+	}
+}