diff --git a/src/core/backend.ts b/src/core/backend.ts
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+++ b/src/core/backend.ts
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+/* =====================================================================
+ * backend.ts — 持久化后端抽象层
+ *
+ * store 只管内存状态 + Op Log，持久化通过 Backend 接口抽象。
+ * 三个实现：
+ *   - LocalBackend   （localStorage，Web 版当前模式）
+ *   - TauriBackend   （Rust 文件系统，桌面版，B4 实现）
+ *   - CloudBackend   （HTTP API，云端版，C0 实现）
+ *
+ * 设计原则：
+ *   - 接口只管读写数据，不管状态逻辑（状态逻辑在 store）
+ *   - 所有方法返回 Promise（TauriBackend/CloudBackend 天然异步）
+ *   - LocalBackend 内部用 localStorage 但包装为 Promise（统一接口）
+ * ===================================================================== */
+import type { Deck, LibItem, PageTemplate, Theme, ChatMessage, AiCfg } from './types'
+
+export interface Backend {
+  // ---- 当前 deck ----
+  loadDeck(): Promise<Deck | null>
+  saveDeck(deck: Deck): Promise<void>
+  loadBackup(): Promise<Deck | null>
+  saveBackup(deck: Deck): Promise<void>
+  clearBackup(): Promise<void>
+
+  // ---- 文库 ----
+  loadLibrary(): Promise<LibItem[]>
+  saveLibrary(lib: LibItem[]): Promise<void>
+  getActiveLibId(): Promise<string | null>
+  setActiveLibId(id: string | null): Promise<void>
+
+  // ---- 会话（按 chatId 隔离） ----
+  loadChat(chatId: string): Promise<ChatMessage[]>
+  saveChat(chatId: string, msgs: ChatMessage[]): Promise<void>
+  clearChat(chatId: string): Promise<void>
+
+  // ---- 用户模板 ----
+  loadTemplates(): Promise<PageTemplate[]>
+  saveTemplates(tpls: PageTemplate[]): Promise<void>
+
+  // ---- 自定义主题 ----
+  loadThemes(): Promise<Record<string, Theme>>
+  saveThemes(themes: Record<string, Theme>): Promise<void>
+
+  // ---- AI 配置 ----
+  loadCfg(): Promise<Partial<AiCfg> | null>
+  saveCfg(cfg: AiCfg): Promise<void>
+}
+
+/* ============================================================
+ * LocalBackend — localStorage 实现（Web 版当前模式）
+ * ============================================================ */
+
+const KEYS = {
+  deck: 'u-ppt.deck.v1',
+  deckBak: 'u-ppt.deck.v1.bak',
+  lib: 'u-ppt.library.v1',
+  activeLib: 'u-ppt.activeLib.v1',
+  chatPrefix: 'u-ppt.chat.',
+  templates: 'u-ppt.templates.v1',
+  themes: 'u-ppt.themes.v1',
+  cfg: 'u-ppt.ai-cfg.v1',
+}
+
+function lsGet(key: string): string | null {
+  try { return localStorage.getItem(key) } catch (e) { return null }
+}
+function lsSet(key: string, val: string): void {
+  try { localStorage.setItem(key, val) } catch (e) { /* 配额满等忽略 */ }
+}
+function lsDel(key: string): void {
+  try { localStorage.removeItem(key) } catch (e) {}
+}
+
+export class LocalBackend implements Backend {
+  async loadDeck(): Promise<Deck | null> {
+    const raw = lsGet(KEYS.deck)
+    if (!raw) return null
+    try { return JSON.parse(raw) } catch (e) { return null }
+  }
+
+  async saveDeck(deck: Deck): Promise<void> {
+    lsSet(KEYS.deck, JSON.stringify(deck))
+  }
+
+  async loadBackup(): Promise<Deck | null> {
+    const raw = lsGet(KEYS.deckBak)
+    if (!raw) return null
+    try { return JSON.parse(raw) } catch (e) { return null }
+  }
+
+  async saveBackup(deck: Deck): Promise<void> {
+    lsSet(KEYS.deckBak, JSON.stringify(deck))
+  }
+
+  async clearBackup(): Promise<void> {
+    lsDel(KEYS.deckBak)
+  }
+
+  async loadLibrary(): Promise<LibItem[]> {
+    const raw = lsGet(KEYS.lib)
+    if (!raw) return []
+    try { return JSON.parse(raw) || [] } catch (e) { return [] }
+  }
+
+  async saveLibrary(lib: LibItem[]): Promise<void> {
+    lsSet(KEYS.lib, JSON.stringify(lib))
+  }
+
+  async getActiveLibId(): Promise<string | null> {
+    return lsGet(KEYS.activeLib)
+  }
+
+  async setActiveLibId(id: string | null): Promise<void> {
+    if (id) lsSet(KEYS.activeLib, id)
+    else lsDel(KEYS.activeLib)
+  }
+
+  async loadChat(chatId: string): Promise<ChatMessage[]> {
+    const raw = lsGet(KEYS.chatPrefix + chatId)
+    if (!raw) return []
+    try { return JSON.parse(raw) || [] } catch (e) { return [] }
+  }
+
+  async saveChat(chatId: string, msgs: ChatMessage[]): Promise<void> {
+    lsSet(KEYS.chatPrefix + chatId, JSON.stringify(msgs))
+  }
+
+  async clearChat(chatId: string): Promise<void> {
+    lsDel(KEYS.chatPrefix + chatId)
+  }
+
+  async loadTemplates(): Promise<PageTemplate[]> {
+    const raw = lsGet(KEYS.templates)
+    if (!raw) return []
+    try { return JSON.parse(raw) || [] } catch (e) { return [] }
+  }
+
+  async saveTemplates(tpls: PageTemplate[]): Promise<void> {
+    // 只存用户模板（category === 'user'）
+    lsSet(KEYS.templates, JSON.stringify(tpls.filter(t => t.category === 'user')))
+  }
+
+  async loadThemes(): Promise<Record<string, Theme>> {
+    const raw = lsGet(KEYS.themes)
+    if (!raw) return {}
+    try { return JSON.parse(raw) || {} } catch (e) { return {} }
+  }
+
+  async saveThemes(themes: Record<string, Theme>): Promise<void> {
+    lsSet(KEYS.themes, JSON.stringify(themes))
+  }
+
+  async loadCfg(): Promise<Partial<AiCfg> | null> {
+    const raw = lsGet(KEYS.cfg)
+    if (!raw) return null
+    try { return JSON.parse(raw) } catch (e) { return null }
+  }
+
+  async saveCfg(cfg: AiCfg): Promise<void> {
+    lsSet(KEYS.cfg, JSON.stringify(cfg))
+  }
+}
+
+/* ---------- 当前活动的 Backend 实例 ---------- */
+let activeBackend: Backend = new LocalBackend()
+
+export function getBackend(): Backend {
+  return activeBackend
+}
+
+export function setBackend(b: Backend): void {
+  activeBackend = b
+}