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/actions/scroll-lock.ts

@@ -0,0 +1,50 @@
+// Body scroll-lock for open dialogs/sheets. While any locker is active, the
+// document body can't scroll behind the overlay. Ref-counted so that nested or
+// stacked dialogs (e.g. a ConfirmSheet opened from the LightboxModal) don't let
+// the first one to close unlock the page out from under the others. Compensates
+// for the vanished scrollbar width so the layout doesn't jump on lock.
+
+import type { ActionReturn } from 'svelte/action';
+
+let lockCount = 0;
+let savedOverflow = '';
+let savedPaddingRight = '';
+
+function lock() {
+	if (typeof document === 'undefined') return;
+	if (lockCount === 0) {
+		const body = document.body;
+		const scrollbarWidth = window.innerWidth - document.documentElement.clientWidth;
+		savedOverflow = body.style.overflow;
+		savedPaddingRight = body.style.paddingRight;
+		body.style.overflow = 'hidden';
+		if (scrollbarWidth > 0) {
+			const current = parseFloat(getComputedStyle(body).paddingRight) || 0;
+			body.style.paddingRight = `${current + scrollbarWidth}px`;
+		}
+	}
+	lockCount++;
+}
+
+function unlock() {
+	if (typeof document === 'undefined') return;
+	lockCount = Math.max(0, lockCount - 1);
+	if (lockCount === 0) {
+		document.body.style.overflow = savedOverflow;
+		document.body.style.paddingRight = savedPaddingRight;
+	}
+}
+
+/**
+ * Locks body scroll for the lifetime of the node. Mount the node only while the
+ * dialog is open (e.g. inside `{#if open}` or alongside a `class:` toggle) so the
+ * action's create/destroy line up with open/close.
+ */
+export function scrollLock(node: HTMLElement): ActionReturn {
+	lock();
+	return {
+		destroy() {
+			unlock();
+		}
+	};
+}