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/CameraCapture.svelte

@@ -48,7 +48,9 @@
 		try {
 			stream = await navigator.mediaDevices.getUserMedia({
 				video: { facingMode, width: { ideal: 1920 }, height: { ideal: 1080 } },
-				audio: true
+				// Only request the mic for video — a photo-only session shouldn't trigger
+				// a mic permission prompt (which also blocks capture on mic-less devices).
+				audio: mode === 'video'
 			});
 			if (videoEl) {
 				videoEl.srcObject = stream;
@@ -78,6 +80,14 @@
 		await startCamera();
 	}
 
+	// Switching between photo and video changes whether we need the mic, so
+	// re-acquire the stream (lazily adding the mic for video, dropping it for photo).
+	async function setMode(next: 'photo' | 'video') {
+		if (mode === next) return;
+		mode = next;
+		await startCamera();
+	}
+
 	function capturePhoto() {
 		if (!videoEl || !canvasEl) return;
 		const ctx = canvasEl.getContext('2d');
@@ -156,12 +166,20 @@
 						<path stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M15 10l-4 4m0-4l4 4m6-4a9 9 0 11-18 0 9 9 0 0118 0z" />
 					</svg>
 					<p class="text-sm text-white">{error}</p>
-					<button
-						onclick={onclose}
-						class="mt-4 rounded-lg bg-white/20 px-4 py-2 text-sm text-white"
-					>
-						Schliessen
-					</button>
+					<div class="mt-4 flex justify-center gap-2">
+						<button
+							onclick={startCamera}
+							class="rounded-lg bg-white px-4 py-2 text-sm font-medium text-gray-900"
+						>
+							Erneut versuchen
+						</button>
+						<button
+							onclick={onclose}
+							class="rounded-lg bg-white/20 px-4 py-2 text-sm text-white"
+						>
+							Schließen
+						</button>
+					</div>
 				</div>
 			</div>
 		{:else}
@@ -198,7 +216,7 @@
 						type="button"
 						role="tab"
 						aria-selected={mode === 'photo'}
-						onclick={() => (mode = 'photo')}
+						onclick={() => setMode('photo')}
 						data-testid="camera-mode-photo"
 						class="rounded-full px-4 py-1 text-sm font-medium transition {mode === 'photo' ? 'bg-white text-gray-900' : 'text-white/70 hover:text-white active:text-white'}"
 					>
@@ -208,7 +226,7 @@
 						type="button"
 						role="tab"
 						aria-selected={mode === 'video'}
-						onclick={() => (mode = 'video')}
+						onclick={() => setMode('video')}
 						data-testid="camera-mode-video"
 						class="rounded-full px-4 py-1 text-sm font-medium transition {mode === 'video' ? 'bg-white text-gray-900' : 'text-white/70 hover:text-white active:text-white'}"
 					>