feat(eventsnap): UX review batch-3 — feed virtualization, a11y/UX fixes, shared primitives

Frontend
- Feed: DOM-windowing via @tanstack/svelte-virtual (new VirtualFeed). The window
  virtualizer keeps the sticky header, pull-to-refresh, infinite-scroll sentinel and
  bottom nav working. List = dynamic measured heights keyed by upload id +
  anchorTo:start so an SSE prepend doesn't jump a scrolled reader; grid = measured
  square rows. Drops the content-visibility band-aid and the old FeedGrid.
  measureElement(null) on row unmount prevents ResizeObserver retention; stable
  option callbacks + guarded setOptions avoid O(n) re-measure on like/comment patches.
- Global: viewport-fit=cover (activates safe-area insets), lang=de, PWA manifest +
  maskable icon + apple-touch metas, prefers-reduced-motion, safe-area-top headers.
- Feed reactivity: like/comment SSE patch the single card in place; upload-processed
  debounced in-place merge; IntersectionObserver leak fixed; shared 60s clock.
- CLS: list cards reserve the skeleton aspect box.
- Destructive actions (promote/demote/unban, gallery release) routed through
  ConfirmSheet (host + admin).
- Export OOM: streamed download via single-use ticket instead of res.blob().
- Shared primitives: IconButton (44px hit area), scrollLock action; UploadSheet a11y.
- Polish: api timeout + non-JSON guard, focus-trap offsetParent fix, pull-to-refresh
  passive:false + guards, diashow keyboard a11y, Toaster assertive errors, dark-mode
  gaps, aria-pressed on like/chip state, CameraCapture mic-on-video + retry,
  ConfirmSheet spinner, upload beforeunload warning, emoji->SVG icons.

Backend
- social.rs: like/comment SSE broadcasts now carry the fresh count so feed clients
  patch one card in place instead of refetching page 1.
- admin.rs / main.rs: supporting changes for the above.

Verified: svelte-check 0 errors, frontend build clean, /feed SSR 200. Runtime feed
scroll-feel validation still owed (documented in FOLLOWUPS.md).

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

frontend/src/lib/components/LightboxModal.svelte

@@ -1,10 +1,12 @@
 <script lang="ts">
+	import { onDestroy } from 'svelte';
 	import type { FeedUpload } from '$lib/types';
 	import { api } from '$lib/api';
 	import { getUserId } from '$lib/auth';
 	import { dataMode, pickMediaUrl } from '$lib/data-mode-store';
 	import { doubletap } from '$lib/actions/doubletap';
 	import { focusTrap } from '$lib/actions/focus-trap';
+	import { scrollLock } from '$lib/actions/scroll-lock';
 	import { toastError } from '$lib/toast-store';
 	import { vibrate } from '$lib/haptics';
 	import HeartBurst from './HeartBurst.svelte';
@@ -33,6 +35,7 @@
 	let loading = $state(false);
 	let userId = getUserId();
 	let heartBurst = $state(false);
+	let burstTimer: ReturnType<typeof setTimeout> | null = null;
 
 	const mediaSrc = $derived(pickMediaUrl($dataMode, upload));
 
@@ -40,9 +43,14 @@
 		heartBurst = true;
 		vibrate(10);
 		onlike(upload.id);
-		setTimeout(() => (heartBurst = false), 700);
+		if (burstTimer) clearTimeout(burstTimer);
+		burstTimer = setTimeout(() => (heartBurst = false), 700);
 	}
 
+	onDestroy(() => {
+		if (burstTimer) clearTimeout(burstTimer);
+	});
+
 	$effect(() => {
 		loadComments();
 	});
@@ -97,6 +105,7 @@
 	aria-modal="true"
 	aria-labelledby="lightbox-title"
 	use:focusTrap={{ onclose }}
+	use:scrollLock
 >
 	<div class="flex max-h-[90vh] w-full max-w-2xl flex-col overflow-hidden rounded-xl bg-white dark:bg-gray-900">
 		<!-- Media -->
@@ -104,7 +113,7 @@
 			<button
 				onclick={onclose}
 				aria-label="Schließen"
-				class="absolute right-2 top-2 z-10 rounded-full bg-black/50 p-1.5 text-white hover:bg-black/70 active:bg-black/70"
+				class="absolute right-2 top-2 z-10 inline-flex min-h-11 min-w-11 items-center justify-center rounded-full bg-black/50 text-white hover:bg-black/70 active:bg-black/70"
 			>
 				<svg class="h-5 w-5" fill="none" viewBox="0 0 24 24" stroke="currentColor">
 					<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12" />