refactor: stores 분리 — toast / uiState 단일 책임화

UX/UI 개편 Phase A-2. lib/stores/ui.ts에 섞여 있던 toast 시스템과
UI layer 상태(미사용 dead export 포함)를 의미 단위로 분리한다.
한 파일이 비대해지는 시나리오를 처음부터 차단(plan 8대 원칙 #7).

- lib/stores/toast.ts 신규 — toasts/addToast/removeToast (Toast interface export)
- lib/stores/uiState.svelte.ts 신규 — drawer 단일 slot + modal stack 클래스 (5대 원칙 #2)
  · openDrawer/closeDrawer/isDrawerOpen
  · openModal/closeTopModal/isModalOpen/modalIndex/topModal
  · handleEscape (modal stack 우선 → drawer)
- lib/stores/ui.ts 삭제 — sidebarOpen/selectedDocId는 어디서도 import되지 않은 dead export였음
- 11개 파일 import 경로 갱신: \$lib/stores/ui → \$lib/stores/toast

uiState는 아직 어디서도 사용 안 함 — Phase B에서 sidebar/meta drawer가 전환될 때
ui.openDrawer('sidebar') 형태로 채택. 동작 변경 0.

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

frontend/src/routes/+layout.svelte

@@ -4,7 +4,7 @@
   import { page } from '$app/stores';
   import { goto } from '$app/navigation';
   import { isAuthenticated, user, tryRefresh, logout } from '$lib/stores/auth';
-  import { toasts, removeToast } from '$lib/stores/ui';
+  import { toasts, removeToast } from '$lib/stores/toast';
   import Sidebar from '$lib/components/Sidebar.svelte';
   import '../app.css';
 

frontend/src/routes/+page.svelte

@@ -2,7 +2,7 @@
   import { onMount } from 'svelte';
   import { goto } from '$app/navigation';
   import { api } from '$lib/api';
-  import { addToast } from '$lib/stores/ui';
+  import { addToast } from '$lib/stores/toast';
 
   let dashboard = null;
   let loading = true;

frontend/src/routes/documents/+page.svelte

@@ -2,7 +2,7 @@
   import { page } from '$app/stores';
   import { goto } from '$app/navigation';
   import { api } from '$lib/api';
-  import { addToast } from '$lib/stores/ui';
+  import { addToast } from '$lib/stores/toast';
   import { Info } from 'lucide-svelte';
   import { List, LayoutGrid } from 'lucide-svelte';
   import DocumentCard from '$lib/components/DocumentCard.svelte';

frontend/src/routes/documents/[id]/+page.svelte

@@ -2,7 +2,7 @@
   import { onMount } from 'svelte';
   import { page } from '$app/stores';
   import { api, getAccessToken } from '$lib/api';
-  import { addToast } from '$lib/stores/ui';
+  import { addToast } from '$lib/stores/toast';
   import { marked } from 'marked';
   import DOMPurify from 'dompurify';
   import TagPill from '$lib/components/TagPill.svelte';

frontend/src/routes/inbox/+page.svelte

@@ -1,7 +1,7 @@
 <script>
   import { onMount } from 'svelte';
   import { api } from '$lib/api';
-  import { addToast } from '$lib/stores/ui';
+  import { addToast } from '$lib/stores/toast';
 
   let documents = [];
   let loading = true;

frontend/src/routes/login/+page.svelte

@@ -1,7 +1,7 @@
 <script>
   import { goto } from '$app/navigation';
   import { login } from '$lib/stores/auth';
-  import { addToast } from '$lib/stores/ui';
+  import { addToast } from '$lib/stores/toast';
 
   let username = '';
   let password = '';

frontend/src/routes/news/+page.svelte

@@ -3,7 +3,7 @@
   import { page } from '$app/stores';
   import { goto } from '$app/navigation';
   import { api } from '$lib/api';
-  import { addToast } from '$lib/stores/ui';
+  import { addToast } from '$lib/stores/toast';
   import { marked } from 'marked';
   import DOMPurify from 'dompurify';
 

frontend/src/routes/settings/+page.svelte

@@ -1,7 +1,7 @@
 <script>
   import { onMount } from 'svelte';
   import { api } from '$lib/api';
-  import { addToast } from '$lib/stores/ui';
+  import { addToast } from '$lib/stores/toast';
   import { user } from '$lib/stores/auth';
 
   let currentPassword = '';