fix: 3+4단계 — 반응형/에러분기/a11y/Synology URL

- DocumentCard: window.innerWidth → matchMedia (반응형 정확)
- documents/[id]: 로딩 상태 3분기 (loading/not_found/network)
- documents/[id]: Synology URL 하드코딩 → edit_url fallback
- DocumentCard: aria-label 추가
- Toast: aria-live 이미 적용 (1단계)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/DocumentCard.svelte

@@ -35,9 +35,14 @@
 
   let domainColor = $derived(DOMAIN_COLORS[doc.ai_domain] || 'var(--border)');
 
+  // 반응형: CSS media query matchMedia 사용
+  let isDesktop = $state(typeof window !== 'undefined' ? window.matchMedia('(min-width: 1024px)').matches : true);
+  if (typeof window !== 'undefined') {
+    window.matchMedia('(min-width: 1024px)').addEventListener('change', (e) => isDesktop = e.matches);
+  }
+
   function handleClick() {
-    // 모바일에서는 항상 detail 페이지로 이동
-    if (window.innerWidth < 1024) {
+    if (!isDesktop) {
       goto(`/documents/${doc.id}`);
       return;
     }
@@ -51,6 +56,7 @@
 
 <button
   onclick={handleClick}
+  aria-label={doc.title || '문서 선택'}
   class="flex items-stretch bg-[var(--surface)] border rounded-lg hover:border-[var(--accent)] transition-colors group w-full text-left overflow-hidden
     {selected ? 'border-[var(--accent)] bg-[var(--accent)]/5' : 'border-[var(--border)]'}"
 >