feat(codex): add opt-in migration and ledger-based restore for unified session history

- Enable dialog gains a checkbox (default off) to migrate existing
  official sessions from the built-in "openai" bucket into the shared
  "custom" bucket, with per-generation backups; failed migrations retry
  at startup
- Disable dialog offers a precise restore driven by the backup ledger:
  only sessions recorded as "openai" in backups are flipped back, and
  sessions created while the toggle was on are never touched
- Completion marker and backup generations are bound to the canonical
  Codex config dir; migrate/restore serialize on an op lock and the
  marker is written conditionally inside the settings write lock
- save_settings rolls back the toggle and fails the save when the live
  rewrite fails; migration additionally requires the live config to
  actually route to the shared bucket (skips with live_not_unified so
  refused injection or proxy takeover can't split history)
- Restore refuses to run while the toggle is (re-)enabled and reports
  nothing_to_restore instead of a zero-count success; local migration
  markers are now backend-owned in merge_settings_for_save so stale
  frontend payloads can't resurrect them
- Settings autosave reverts optimistic form state on failure so a
  failed toggle change can't be replayed by an unrelated save
- ConfirmDialog supports an optional checkbox; all four locales updated

src/lib/api/settings.ts

@@ -19,6 +19,13 @@ export interface WebDavTestResult {
   message?: string;
 }
 
+export interface CodexUnifyHistoryRestoreResult {
+  restoredJsonlFiles: number;
+  restoredStateRows: number;
+  /** 还原被跳过的原因（如当前目录没有账本）；存在时不应报成功 */
+  skippedReason?: string;
+}
+
 export interface WebDavSyncResult {
   status: string;
 }
@@ -32,6 +39,16 @@ export const settingsApi = {
     return await invoke("save_settings", { settings });
   },
 
+  /** 是否存在统一 Codex 会话历史的迁移备份（关闭弹窗据此显示"恢复备份"勾选） */
+  async hasCodexUnifyHistoryBackup(): Promise<boolean> {
+    return await invoke("has_codex_unify_history_backup");
+  },
+
+  /** 按迁移备份账本把当时迁入共享桶的官方会话还原回 openai 桶（幂等） */
+  async restoreCodexUnifiedHistory(): Promise<CodexUnifyHistoryRestoreResult> {
+    return await invoke("restore_codex_unified_history");
+  },
+
   async restart(): Promise<boolean> {
     return await invoke("restart_app");
   },