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/UploadSheet.svelte

@@ -2,11 +2,14 @@
 	import { goto } from '$app/navigation';
 	import { uploadSheetOpen } from '$lib/ui-store';
 	import { pendingFiles } from '$lib/pending-upload-store';
+	import { scrollLock } from '$lib/actions/scroll-lock';
 	import CameraCapture from '$lib/components/CameraCapture.svelte';
 	import type { PendingFile } from '$lib/pending-upload-store';
 
 	let showCamera = $state(false);
 	let fileInput: HTMLInputElement;
+	let sheet = $state<HTMLDivElement | null>(null);
+	let returnFocus: HTMLElement | null = null;
 
 	// Keep the sheet and backdrop always in the DOM for smooth CSS transitions.
 	let open = $derived($uploadSheetOpen);
@@ -15,6 +18,47 @@
 		uploadSheetOpen.set(false);
 	}
 
+	// Focus-trap + Escape, wired manually because the sheet stays mounted for its
+	// translate-y animation (so use:focusTrap, which activates on mount, won't do).
+	// Mirrors ContextSheet. Suspended while the camera overlay owns the screen.
+	function onKeyDown(e: KeyboardEvent) {
+		if (showCamera) return;
+		if (e.key === 'Escape') {
+			e.preventDefault();
+			close();
+			return;
+		}
+		if (e.key !== 'Tab' || !sheet) return;
+		const list = Array.from(sheet.querySelectorAll<HTMLElement>('button:not([disabled])'));
+		if (list.length === 0) return;
+		const first = list[0];
+		const last = list[list.length - 1];
+		const active = document.activeElement as HTMLElement | null;
+		if (e.shiftKey && (active === first || !sheet.contains(active))) {
+			e.preventDefault();
+			last.focus();
+		} else if (!e.shiftKey && active === last) {
+			e.preventDefault();
+			first.focus();
+		}
+	}
+
+	$effect(() => {
+		if (open) {
+			returnFocus = (document.activeElement as HTMLElement | null) ?? null;
+			requestAnimationFrame(() => {
+				if (showCamera) return;
+				const first = sheet?.querySelector<HTMLButtonElement>('button:not([disabled])');
+				first?.focus({ preventScroll: true });
+			});
+			window.addEventListener('keydown', onKeyDown);
+			return () => window.removeEventListener('keydown', onKeyDown);
+		} else if (returnFocus) {
+			try { returnFocus.focus({ preventScroll: true }); } catch { /* element gone */ }
+			returnFocus = null;
+		}
+	});
+
 	function openGallery() {
 		fileInput?.click();
 	}
@@ -68,22 +112,34 @@
 	onchange={handleFiles}
 />
 
-<!-- Backdrop -->
-<div
+<!-- Lock body scroll only while open (sheet stays mounted for its animation). -->
+{#if open && !showCamera}
+	<div use:scrollLock class="hidden"></div>
+{/if}
+
+<!-- Backdrop — real <button> so keyboard / switch-control users get parity. -->
+<button
+	type="button"
 	class="fixed inset-0 z-40 bg-black/50 transition-opacity duration-300"
 	class:opacity-0={!open}
 	class:pointer-events-none={!open}
 	class:opacity-100={open}
 	onclick={close}
-	aria-hidden="true"
-></div>
+	tabindex="-1"
+	aria-label="Schließen"
+></button>
 
 <!-- Sheet -->
 <div
+	bind:this={sheet}
 	class="fixed inset-x-0 bottom-0 z-50 rounded-t-2xl bg-white transition-transform duration-300 dark:bg-gray-900"
 	class:translate-y-full={!open}
 	class:translate-y-0={open}
 	style="padding-bottom: env(safe-area-inset-bottom)"
+	role="dialog"
+	aria-modal="true"
+	aria-label="Hochladen"
+	tabindex="-1"
 >
 	<!-- Drag handle -->
 	<div class="flex justify-center pt-3 pb-1">