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>

backend/src/handlers/admin.rs

@@ -1,7 +1,7 @@
 use std::collections::HashMap;
 use std::time::Duration;
 
-use axum::extract::State;
+use axum::extract::{Query, State};
 use axum::http::{HeaderMap, StatusCode};
 use axum::Json;
 use serde::{Deserialize, Serialize};
@@ -223,11 +223,44 @@ pub async fn get_export_jobs(
 
 // ── Export download endpoints (authenticated guests) ─────────────────────────
 
+#[derive(Deserialize)]
+pub struct DownloadQuery {
+    pub ticket: String,
+}
+
+/// Mint a short-lived ticket for a browser-driven export download. The download
+/// is a top-level navigation so the multi-GB ZIP streams straight to disk instead
+/// of being buffered in memory by `fetch()` + `blob()` — but a navigation can't
+/// carry an `Authorization` header, so the client exchanges its Bearer token for
+/// an opaque ticket here, then hits `/export/zip?ticket=...`. Reuses the same
+/// single-use, 30s-TTL store as the SSE stream.
+pub async fn export_ticket(
+    State(state): State<AppState>,
+    auth: crate::auth::middleware::AuthUser,
+) -> Json<serde_json::Value> {
+    let ticket = state.sse_tickets.issue(auth.token_hash);
+    Json(serde_json::json!({ "ticket": ticket }))
+}
+
+/// Validate a download ticket (single-use) and confirm its session still exists.
+async fn authenticate_download_ticket(state: &AppState, ticket: &str) -> Result<(), AppError> {
+    let token_hash = state
+        .sse_tickets
+        .consume(ticket)
+        .ok_or_else(|| AppError::Unauthorized("Ticket ungültig oder abgelaufen.".into()))?;
+    crate::models::session::Session::find_by_token_hash(&state.pool, &token_hash)
+        .await
+        .map_err(|e| AppError::Internal(e.into()))?
+        .ok_or_else(|| AppError::Unauthorized("Sitzung nicht gefunden.".into()))?;
+    Ok(())
+}
+
 pub async fn download_zip(
     State(state): State<AppState>,
-    _auth: crate::auth::middleware::AuthUser,
+    Query(q): Query<DownloadQuery>,
     headers: HeaderMap,
 ) -> Result<axum::response::Response, AppError> {
+    authenticate_download_ticket(&state, &q.ticket).await?;
     enforce_export_rate(&state, &headers).await?;
 
     let event = crate::models::event::Event::find_by_slug(&state.pool, &state.config.event_slug)
@@ -250,9 +283,10 @@ pub async fn download_zip(
 
 pub async fn download_html(
     State(state): State<AppState>,
-    _auth: crate::auth::middleware::AuthUser,
+    Query(q): Query<DownloadQuery>,
     headers: HeaderMap,
 ) -> Result<axum::response::Response, AppError> {
+    authenticate_download_ticket(&state, &q.ticket).await?;
     enforce_export_rate(&state, &headers).await?;
 
     let event = crate::models::event::Event::find_by_slug(&state.pool, &state.config.event_slug)