fix: 프론트엔드 1단계 — XSS 수정 + Svelte 5 변환 + 필터/아이콘/a11y

- [critical] DOMPurify 적용 (FORBID_TAGS/ATTR, ALLOW_UNKNOWN_PROTOCOLS)
- [high] $: → $derived 변환 (documents/[id])
- [high] 태그/소스 필터 구현 (filterTag, filterSource)
- FormatIcon: docx/xlsx/pptx/odt/ods/odp/dwg/dxf 추가
- editTab 선언 순서 수정
- debounceTimer 미사용 변수 제거
- Toast role="status" aria-live 추가
- marked 옵션: mangle/headerIds false

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

frontend/src/routes/+layout.svelte

@@ -113,7 +113,7 @@
 {/if}
 
 <!-- Toast -->
-<div class="fixed top-4 right-4 z-50 flex flex-col gap-2 max-w-sm">
+<div class="fixed top-4 right-4 z-50 flex flex-col gap-2 max-w-sm" role="status" aria-live="polite">
   {#each $toasts as toast (toast.id)}
     <button
       class="px-4 py-3 rounded-lg shadow-lg text-sm flex items-center gap-2 cursor-pointer text-left"