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/OnboardingGuide.svelte

@@ -3,6 +3,7 @@
 	import { privacyNote } from '$lib/privacy-note-store';
 	import { themePreference, type ThemePreference } from '$lib/theme-store';
 	import { focusTrap } from '$lib/actions/focus-trap';
+	import { scrollLock } from '$lib/actions/scroll-lock';
 	import { vibrate } from '$lib/haptics';
 
 	const GUIDE_SEEN_KEY = 'eventsnap_guide_seen';
@@ -100,6 +101,7 @@
 			aria-modal="true"
 			aria-labelledby="onboarding-title"
 			use:focusTrap={{ onclose: dismiss }}
+			use:scrollLock
 		>
 			<!-- Step indicator — tap a pip to jump back. The visible dot is small but
 			     the touch target is padded to ~44 px so it remains tappable on mobile. -->